Network Working Group R. Stewart
Request for Comments: 2960 Q. Xie
Category: Standards Track Motorola
K. Morneault
C. Sharp
Cisco
H. Schwarzbauer
Siemens
T. Taylor
Nortel Networks
I. Rytina
Ericsson
M. Kalla
Telcordia
L. Zhang
UCLA
V. Paxson
ACIRI
October 2000
Stream Control Transmission Protocol
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document describes the Stream Control Transmission Protocol
(SCTP). SCTP is designed to transport PSTN signaling messages over
IP networks, but is capable of broader applications.
SCTP is a reliable transport protocol operating on top of a
connectionless packet network such as IP. It offers the following
services to its users:
-- acknowledged error-free non-duplicated transfer of user data,
-- data fragmentation to conform to discovered path MTU size,
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RFC 2960 Stream Control Transmission Protocol October 2000
-- sequenced delivery of user messages within multiple streams,
with an option for order-of-arrival delivery of individual user
messages,
-- optional bundling of multiple user messages into a single SCTP
packet, and
-- network-level fault tolerance through supporting of multi-
homing at either or both ends of an association.
The design of SCTP includes appropriate congestion avoidance behavior
and resistance to flooding and masquerade attacks.
Stewart, et al. Standards Track [Page 2]

RFC 2960 Stream Control Transmission Protocol October 20001.1 Motivation
TCP [RFC793] has performed immense service as the primary means of
reliable data transfer in IP networks. However, an increasing number
of recent applications have found TCP too limiting, and have
incorporated their own reliable data transfer protocol on top of UDP
[RFC768]. The limitations which users have wished to bypass include
the following:
-- TCP provides both reliable data transfer and strict order-of-
transmission delivery of data. Some applications need reliable
transfer without sequence maintenance, while others would be
satisfied with partial ordering of the data. In both of these
cases the head-of-line blocking offered by TCP causes unnecessary
delay.
-- The stream-oriented nature of TCP is often an inconvenience.
Applications must add their own record marking to delineate their
messages, and must make explicit use of the push facility to
ensure that a complete message is transferred in a reasonable
time.
-- The limited scope of TCP sockets complicates the task of
providing highly-available data transfer capability using multi-
homed hosts.
-- TCP is relatively vulnerable to denial of service attacks, such
as SYN attacks.
Transport of PSTN signaling across the IP network is an application
for which all of these limitations of TCP are relevant. While this
application directly motivated the development of SCTP, other
applications may find SCTP a good match to their requirements.
1.2 Architectural View of SCTP
SCTP is viewed as a layer between the SCTP user application ("SCTP
user" for short) and a connectionless packet network service such as
IP. The remainder of this document assumes SCTP runs on top of IP.
The basic service offered by SCTP is the reliable transfer of user
messages between peer SCTP users. It performs this service within
the context of an association between two SCTP endpoints. Section 10
of this document sketches the API which should exist at the boundary
between the SCTP and the SCTP user layers.
SCTP is connection-oriented in nature, but the SCTP association is a
broader concept than the TCP connection. SCTP provides the means for
each SCTP endpoint (Section 1.4) to provide the other endpoint
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RFC 2960 Stream Control Transmission Protocol October 2000
(during association startup) with a list of transport addresses
(i.e., multiple IP addresses in combination with an SCTP port)
through which that endpoint can be reached and from which it will
originate SCTP packets. The association spans transfers over all of
the possible source/destination combinations which may be generated
from each endpoint's lists.
_____________ _____________
| SCTP User | | SCTP User |
| Application | | Application |
|-------------| |-------------|
| SCTP | | SCTP |
| Transport | | Transport |
| Service | | Service |
|-------------| |-------------|
| |One or more ---- One or more| |
| IP Network |IP address \/ IP address| IP Network |
| Service |appearances /\ appearances| Service |
|_____________| ---- |_____________|
SCTP Node A |<-------- Network transport ------->| SCTP Node B
Figure 1: An SCTP Association
1.3 Functional View of SCTP
The SCTP transport service can be decomposed into a number of
functions. These are depicted in Figure 2 and explained in the
remainder of this section.
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RFC 2960 Stream Control Transmission Protocol October 2000
primitive) or as a result of an error condition detected within the
SCTP layer. Section 9 describes both the graceful and the ungraceful
close procedures.
SCTP does not support a half-open state (like TCP) wherein one side
may continue sending data while the other end is closed. When either
endpoint performs a shutdown, the association on each peer will stop
accepting new data from its user and only deliver data in queue at
the time of the graceful close (see Section 9).
1.3.2 Sequenced Delivery within Streams
The term "stream" is used in SCTP to refer to a sequence of user
messages that are to be delivered to the upper-layer protocol in
order with respect to other messages within the same stream. This is
in contrast to its usage in TCP, where it refers to a sequence of
bytes (in this document a byte is assumed to be eight bits).
The SCTP user can specify at association startup time the number of
streams to be supported by the association. This number is
negotiated with the remote end (see Section 5.1.1). User messages
are associated with stream numbers (SEND, RECEIVE primitives, Section10). Internally, SCTP assigns a stream sequence number to each
message passed to it by the SCTP user. On the receiving side, SCTP
ensures that messages are delivered to the SCTP user in sequence
within a given stream. However, while one stream may be blocked
waiting for the next in-sequence user message, delivery from other
streams may proceed.
SCTP provides a mechanism for bypassing the sequenced delivery
service. User messages sent using this mechanism are delivered to
the SCTP user as soon as they are received.
1.3.3 User Data Fragmentation
When needed, SCTP fragments user messages to ensure that the SCTP
packet passed to the lower layer conforms to the path MTU. On
receipt, fragments are reassembled into complete messages before
being passed to the SCTP user.
1.3.4 Acknowledgement and Congestion Avoidance
SCTP assigns a Transmission Sequence Number (TSN) to each user data
fragment or unfragmented message. The TSN is independent of any
stream sequence number assigned at the stream level. The receiving
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RFC 2960 Stream Control Transmission Protocol October 2000
end acknowledges all TSNs received, even if there are gaps in the
sequence. In this way, reliable delivery is kept functionally
separate from sequenced stream delivery.
The acknowledgement and congestion avoidance function is responsible
for packet retransmission when timely acknowledgement has not been
received. Packet retransmission is conditioned by congestion
avoidance procedures similar to those used for TCP. See Sections 6
and 7 for a detailed description of the protocol procedures
associated with this function.
1.3.5 Chunk Bundling
As described in Section 3, the SCTP packet as delivered to the lower
layer consists of a common header followed by one or more chunks.
Each chunk may contain either user data or SCTP control information.
The SCTP user has the option to request bundling of more than one
user messages into a single SCTP packet. The chunk bundling function
of SCTP is responsible for assembly of the complete SCTP packet and
its disassembly at the receiving end.
During times of congestion an SCTP implementation MAY still perform
bundling even if the user has requested that SCTP not bundle. The
user's disabling of bundling only affects SCTP implementations that
may delay a small period of time before transmission (to attempt to
encourage bundling). When the user layer disables bundling, this
small delay is prohibited but not bundling that is performed during
congestion or retransmission.
1.3.6 Packet Validation
A mandatory Verification Tag field and a 32 bit checksum field (see
Appendix B for a description of the Adler-32 checksum) are included
in the SCTP common header. The Verification Tag value is chosen by
each end of the association during association startup. Packets
received without the expected Verification Tag value are discarded,
as a protection against blind masquerade attacks and against stale
SCTP packets from a previous association. The Adler-32 checksum
should be set by the sender of each SCTP packet to provide additional
protection against data corruption in the network. The receiver of
an SCTP packet with an invalid Adler-32 checksum silently discards
the packet.
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RFC 2960 Stream Control Transmission Protocol October 20001.3.7 Path Management
The sending SCTP user is able to manipulate the set of transport
addresses used as destinations for SCTP packets through the
primitives described in Section 10. The SCTP path management
function chooses the destination transport address for each outgoing
SCTP packet based on the SCTP user's instructions and the currently
perceived reachability status of the eligible destination set. The
path management function monitors reachability through heartbeats
when other packet traffic is inadequate to provide this information
and advises the SCTP user when reachability of any far-end transport
address changes. The path management function is also responsible
for reporting the eligible set of local transport addresses to the
far end during association startup, and for reporting the transport
addresses returned from the far end to the SCTP user.
At association start-up, a primary path is defined for each SCTP
endpoint, and is used for normal sending of SCTP packets.
On the receiving end, the path management is responsible for
verifying the existence of a valid SCTP association to which the
inbound SCTP packet belongs before passing it for further processing.
Note: Path Management and Packet Validation are done at the same
time, so although described separately above, in reality they cannot
be performed as separate items.
1.4 Key Terms
Some of the language used to describe SCTP has been introduced in the
previous sections. This section provides a consolidated list of the
key terms and their definitions.
o Active destination transport address: A transport address on a
peer endpoint which a transmitting endpoint considers available
for receiving user messages.
o Bundling: An optional multiplexing operation, whereby more than
one user message may be carried in the same SCTP packet. Each
user message occupies its own DATA chunk.
o Chunk: A unit of information within an SCTP packet, consisting of
a chunk header and chunk-specific content.
o Congestion Window (cwnd): An SCTP variable that limits the data,
in number of bytes, a sender can send to a particular destination
transport address before receiving an acknowledgement.
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RFC 2960 Stream Control Transmission Protocol October 2000
o Cumulative TSN Ack Point: The TSN of the last DATA chunk
acknowledged via the Cumulative TSN Ack field of a SACK.
o Idle destination address: An address that has not had user
messages sent to it within some length of time, normally the
HEARTBEAT interval or greater.
o Inactive destination transport address: An address which is
considered inactive due to errors and unavailable to transport
user messages.
o Message = user message: Data submitted to SCTP by the Upper Layer
Protocol (ULP).
o Message Authentication Code (MAC): An integrity check mechanism
based on cryptographic hash functions using a secret key.
Typically, message authentication codes are used between two
parties that share a secret key in order to validate information
transmitted between these parties. In SCTP it is used by an
endpoint to validate the State Cookie information that is returned
from the peer in the COOKIE ECHO chunk. The term "MAC" has
different meanings in different contexts. SCTP uses this term
with the same meaning as in [RFC2104].
o Network Byte Order: Most significant byte first, a.k.a., Big
Endian.
o Ordered Message: A user message that is delivered in order with
respect to all previous user messages sent within the stream the
message was sent on.
o Outstanding TSN (at an SCTP endpoint): A TSN (and the associated
DATA chunk) that has been sent by the endpoint but for which it
has not yet received an acknowledgement.
o Path: The route taken by the SCTP packets sent by one SCTP
endpoint to a specific destination transport address of its peer
SCTP endpoint. Sending to different destination transport
addresses does not necessarily guarantee getting separate paths.
o Primary Path: The primary path is the destination and source
address that will be put into a packet outbound to the peer
endpoint by default. The definition includes the source address
since an implementation MAY wish to specify both destination and
source address to better control the return path taken by reply
chunks and on which interface the packet is transmitted when the
data sender is multi-homed.
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RFC 2960 Stream Control Transmission Protocol October 2000
o Receiver Window (rwnd): An SCTP variable a data sender uses to
store the most recently calculated receiver window of its peer, in
number of bytes. This gives the sender an indication of the space
available in the receiver's inbound buffer.
o SCTP association: A protocol relationship between SCTP endpoints,
composed of the two SCTP endpoints and protocol state information
including Verification Tags and the currently active set of
Transmission Sequence Numbers (TSNs), etc. An association can be
uniquely identified by the transport addresses used by the
endpoints in the association. Two SCTP endpoints MUST NOT have
more than one SCTP association between them at any given time.
o SCTP endpoint: The logical sender/receiver of SCTP packets. On a
multi-homed host, an SCTP endpoint is represented to its peers as
a combination of a set of eligible destination transport addresses
to which SCTP packets can be sent and a set of eligible source
transport addresses from which SCTP packets can be received. All
transport addresses used by an SCTP endpoint must use the same
port number, but can use multiple IP addresses. A transport
address used by an SCTP endpoint must not be used by another SCTP
endpoint. In other words, a transport address is unique to an
SCTP endpoint.
o SCTP packet (or packet): The unit of data delivery across the
interface between SCTP and the connectionless packet network
(e.g., IP). An SCTP packet includes the common SCTP header,
possible SCTP control chunks, and user data encapsulated within
SCTP DATA chunks.
o SCTP user application (SCTP user): The logical higher-layer
application entity which uses the services of SCTP, also called
the Upper-layer Protocol (ULP).
o Slow Start Threshold (ssthresh): An SCTP variable. This is the
threshold which the endpoint will use to determine whether to
perform slow start or congestion avoidance on a particular
destination transport address. Ssthresh is in number of bytes.
o Stream: A uni-directional logical channel established from one to
another associated SCTP endpoint, within which all user messages
are delivered in sequence except for those submitted to the
unordered delivery service.
Note: The relationship between stream numbers in opposite directions
is strictly a matter of how the applications use them. It is the
responsibility of the SCTP user to create and manage these
correlations if they are so desired.
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RFC 2960 Stream Control Transmission Protocol October 2000
o Stream Sequence Number: A 16-bit sequence number used internally
by SCTP to assure sequenced delivery of the user messages within a
given stream. One stream sequence number is attached to each user
message.
o Tie-Tags: Verification Tags from a previous association. These
Tags are used within a State Cookie so that the newly restarting
association can be linked to the original association within the
endpoint that did not restart.
o Transmission Control Block (TCB): An internal data structure
created by an SCTP endpoint for each of its existing SCTP
associations to other SCTP endpoints. TCB contains all the status
and operational information for the endpoint to maintain and
manage the corresponding association.
o Transmission Sequence Number (TSN): A 32-bit sequence number used
internally by SCTP. One TSN is attached to each chunk containing
user data to permit the receiving SCTP endpoint to acknowledge its
receipt and detect duplicate deliveries.
o Transport address: A Transport Address is traditionally defined
by Network Layer address, Transport Layer protocol and Transport
Layer port number. In the case of SCTP running over IP, a
transport address is defined by the combination of an IP address
and an SCTP port number (where SCTP is the Transport protocol).
o Unacknowledged TSN (at an SCTP endpoint): A TSN (and the associated
DATA chunk) which has been received by the endpoint but for which
an acknowledgement has not yet been sent. Or in the opposite case,
for a packet that has been sent but no acknowledgement has been
received.
o Unordered Message: Unordered messages are "unordered" with respect
to any other message, this includes both other unordered messages
as well as other ordered messages. Unordered message might be
delivered prior to or later than ordered messages sent on the same
stream.
o User message: The unit of data delivery across the interface
between SCTP and its user.
o Verification Tag: A 32 bit unsigned integer that is randomly
generated. The Verification Tag provides a key that allows a
receiver to verify that the SCTP packet belongs to the current
association and is not an old or stale packet from a previous
association.
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RFC 2960 Stream Control Transmission Protocol October 20001.5. Abbreviations
MAC - Message Authentication Code [RFC2104]
RTO - Retransmission Time-out
RTT - Round-trip Time
RTTVAR - Round-trip Time Variation
SCTP - Stream Control Transmission Protocol
SRTT - Smoothed RTT
TCB - Transmission Control Block
TLV - Type-Length-Value Coding Format
TSN - Transmission Sequence Number
ULP - Upper-layer Protocol
1.6 Serial Number Arithmetic
It is essential to remember that the actual Transmission Sequence
Number space is finite, though very large. This space ranges from 0
to 2**32 - 1. Since the space is finite, all arithmetic dealing with
Transmission Sequence Numbers must be performed modulo 2**32. This
unsigned arithmetic preserves the relationship of sequence numbers as
they cycle from 2**32 - 1 to 0 again. There are some subtleties to
computer modulo arithmetic, so great care should be taken in
programming the comparison of such values. When referring to TSNs,
the symbol "=<" means "less than or equal"(modulo 2**32).
Comparisons and arithmetic on TSNs in this document SHOULD use Serial
Number Arithmetic as defined in [RFC1982] where SERIAL_BITS = 32.
An endpoint SHOULD NOT transmit a DATA chunk with a TSN that is more
than 2**31 - 1 above the beginning TSN of its current send window.
Doing so will cause problems in comparing TSNs.
Transmission Sequence Numbers wrap around when they reach 2**32 - 1.
That is, the next TSN a DATA chunk MUST use after transmitting TSN =
2*32 - 1 is TSN = 0.
Any arithmetic done on Stream Sequence Numbers SHOULD use Serial
Number Arithmetic as defined in [RFC1982] where SERIAL_BITS = 16.
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RFC 2960 Stream Control Transmission Protocol October 2000
All other arithmetic and comparisons in this document uses normal
arithmetic.
2. Conventions
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
they appear in this document, are to be interpreted as described in
[RFC2119].
3. SCTP packet Format
An SCTP packet is composed of a common header and chunks. A chunk
contains either control information or user data.
The SCTP packet format is shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Common Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Chunk #1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Chunk #n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Multiple chunks can be bundled into one SCTP packet up to the MTU
size, except for the INIT, INIT ACK, and SHUTDOWN COMPLETE chunks.
These chunks MUST NOT be bundled with any other chunk in a packet.
See Section 6.10 for more details on chunk bundling.
If a user data message doesn't fit into one SCTP packet it can be
fragmented into multiple chunks using the procedure defined in
Section 6.9.
All integer fields in an SCTP packet MUST be transmitted in network
byte order, unless otherwise stated.
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RFC 2960 Stream Control Transmission Protocol October 20003.1 SCTP Common Header Field Descriptions
SCTP Common Header Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port Number | Destination Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Verification Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Source Port Number: 16 bits (unsigned integer)
This is the SCTP sender's port number. It can be used by the
receiver in combination with the source IP address, the SCTP
destination port and possibly the destination IP address to
identify the association to which this packet belongs.
Destination Port Number: 16 bits (unsigned integer)
This is the SCTP port number to which this packet is destined.
The receiving host will use this port number to de-multiplex the
SCTP packet to the correct receiving endpoint/application.
Verification Tag: 32 bits (unsigned integer)
The receiver of this packet uses the Verification Tag to validate
the sender of this SCTP packet. On transmit, the value of this
Verification Tag MUST be set to the value of the Initiate Tag
received from the peer endpoint during the association
initialization, with the following exceptions:
- A packet containing an INIT chunk MUST have a zero Verification
Tag.
- A packet containing a SHUTDOWN-COMPLETE chunk with the T-bit
set MUST have the Verification Tag copied from the packet with
the SHUTDOWN-ACK chunk.
- A packet containing an ABORT chunk may have the verification
tag copied from the packet which caused the ABORT to be sent.
For details see Section 8.4 and 8.5.
An INIT chunk MUST be the only chunk in the SCTP packet carrying it.
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RFC 2960 Stream Control Transmission Protocol October 2000
14 - Shutdown Complete (SHUTDOWN COMPLETE)
15 to 62 - reserved by IETF
63 - IETF-defined Chunk Extensions
64 to 126 - reserved by IETF
127 - IETF-defined Chunk Extensions
128 to 190 - reserved by IETF
191 - IETF-defined Chunk Extensions
192 to 254 - reserved by IETF
255 - IETF-defined Chunk Extensions
Chunk Types are encoded such that the highest-order two bits specify
the action that must be taken if the processing endpoint does not
recognize the Chunk Type.
00 - Stop processing this SCTP packet and discard it, do not process
any further chunks within it.
01 - Stop processing this SCTP packet and discard it, do not process
any further chunks within it, and report the unrecognized
parameter in an 'Unrecognized Parameter Type' (in either an
ERROR or in the INIT ACK).
10 - Skip this chunk and continue processing.
11 - Skip this chunk and continue processing, but report in an ERROR
Chunk using the 'Unrecognized Chunk Type' cause of error.
Note: The ECNE and CWR chunk types are reserved for future use of
Explicit Congestion Notification (ECN).
Chunk Flags: 8 bits
The usage of these bits depends on the chunk type as given by the
Chunk Type. Unless otherwise specified, they are set to zero on
transmit and are ignored on receipt.
Chunk Length: 16 bits (unsigned integer)
This value represents the size of the chunk in bytes including the
Chunk Type, Chunk Flags, Chunk Length, and Chunk Value fields.
Therefore, if the Chunk Value field is zero-length, the Length
field will be set to 4. The Chunk Length field does not count any
padding.
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RFC 2960 Stream Control Transmission Protocol October 2000
Chunk Value: variable length
The Chunk Value field contains the actual information to be
transferred in the chunk. The usage and format of this field is
dependent on the Chunk Type.
The total length of a chunk (including Type, Length and Value fields)
MUST be a multiple of 4 bytes. If the length of the chunk is not a
multiple of 4 bytes, the sender MUST pad the chunk with all zero
bytes and this padding is not included in the chunk length field.
The sender should never pad with more than 3 bytes. The receiver
MUST ignore the padding bytes.
SCTP defined chunks are described in detail in Section 3.3. The
guidelines for IETF-defined chunk extensions can be found in Section13.1 of this document.
3.2.1 Optional/Variable-length Parameter Format
Chunk values of SCTP control chunks consist of a chunk-type-specific
header of required fields, followed by zero or more parameters. The
optional and variable-length parameters contained in a chunk are
defined in a Type-Length-Value format as shown below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter Type | Parameter Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Parameter Value /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Parameter Type: 16 bits (unsigned integer)
The Type field is a 16 bit identifier of the type of parameter.
It takes a value of 0 to 65534.
The value of 65535 is reserved for IETF-defined extensions. Values
other than those defined in specific SCTP chunk description are
reserved for use by IETF.
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RFC 2960 Stream Control Transmission Protocol October 2000
Chunk Parameter Length: 16 bits (unsigned integer)
The Parameter Length field contains the size of the parameter in
bytes, including the Parameter Type, Parameter Length, and
Parameter Value fields. Thus, a parameter with a zero-length
Parameter Value field would have a Length field of 4. The
Parameter Length does not include any padding bytes.
Chunk Parameter Value: variable-length.
The Parameter Value field contains the actual information to be
transferred in the parameter.
The total length of a parameter (including Type, Parameter Length and
Value fields) MUST be a multiple of 4 bytes. If the length of the
parameter is not a multiple of 4 bytes, the sender pads the Parameter
at the end (i.e., after the Parameter Value field) with all zero
bytes. The length of the padding is not included in the parameter
length field. A sender SHOULD NOT pad with more than 3 bytes. The
receiver MUST ignore the padding bytes.
The Parameter Types are encoded such that the highest-order two bits
specify the action that must be taken if the processing endpoint does
not recognize the Parameter Type.
00 - Stop processing this SCTP packet and discard it, do not process
any further chunks within it.
01 - Stop processing this SCTP packet and discard it, do not process
any further chunks within it, and report the unrecognized
parameter in an 'Unrecognized Parameter Type' (in either an
ERROR or in the INIT ACK).
10 - Skip this parameter and continue processing.
11 - Skip this parameter and continue processing but report the
unrecognized parameter in an 'Unrecognized Parameter Type' (in
either an ERROR or in the INIT ACK).
The actual SCTP parameters are defined in the specific SCTP chunk
sections. The rules for IETF-defined parameter extensions are
defined in Section 13.2.
3.3 SCTP Chunk Definitions
This section defines the format of the different SCTP chunk types.
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RFC 2960 Stream Control Transmission Protocol October 2000
An unfragmented user message shall have both the B and E bits set to
'1'. Setting both B and E bits to '0' indicates a middle fragment of
a multi-fragment user message, as summarized in the following table:
B E Description
============================================================
| 1 0 | First piece of a fragmented user message |
+----------------------------------------------------------+
| 0 0 | Middle piece of a fragmented user message |
+----------------------------------------------------------+
| 0 1 | Last piece of a fragmented user message |
+----------------------------------------------------------+
| 1 1 | Unfragmented Message |
============================================================
| Table 1: Fragment Description Flags |
============================================================
When a user message is fragmented into multiple chunks, the TSNs are
used by the receiver to reassemble the message. This means that the
TSNs for each fragment of a fragmented user message MUST be strictly
sequential.
Length: 16 bits (unsigned integer)
This field indicates the length of the DATA chunk in bytes from
the beginning of the type field to the end of the user data field
excluding any padding. A DATA chunk with no user data field will
have Length set to 16 (indicating 16 bytes).
TSN : 32 bits (unsigned integer)
This value represents the TSN for this DATA chunk. The valid
range of TSN is from 0 to 4294967295 (2**32 - 1). TSN wraps back
to 0 after reaching 4294967295.
Stream Identifier S: 16 bits (unsigned integer)
Identifies the stream to which the following user data belongs.
Stream Sequence Number n: 16 bits (unsigned integer)
This value represents the stream sequence number of the following
user data within the stream S. Valid range is 0 to 65535.
When a user message is fragmented by SCTP for transport, the same
stream sequence number MUST be carried in each of the fragments of
the message.
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RFC 2960 Stream Control Transmission Protocol October 2000
Payload Protocol Identifier: 32 bits (unsigned integer)
This value represents an application (or upper layer) specified
protocol identifier. This value is passed to SCTP by its upper
layer and sent to its peer. This identifier is not used by SCTP
but can be used by certain network entities as well as the peer
application to identify the type of information being carried in
this DATA chunk. This field must be sent even in fragmented DATA
chunks (to make sure it is available for agents in the middle of
the network).
The value 0 indicates no application identifier is specified by
the upper layer for this payload data.
User Data: variable length
This is the payload user data. The implementation MUST pad the
end of the data to a 4 byte boundary with all-zero bytes. Any
padding MUST NOT be included in the length field. A sender MUST
never add more than 3 bytes of padding.
3.3.2 Initiation (INIT) (1)
This chunk is used to initiate a SCTP association between two
endpoints. The format of the INIT chunk is shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 | Chunk Flags | Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiate Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Receiver Window Credit (a_rwnd) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Outbound Streams | Number of Inbound Streams |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Optional/Variable-Length Parameters /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The INIT chunk contains the following parameters. Unless otherwise
noted, each parameter MUST only be included once in the INIT chunk.
Stewart, et al. Standards Track [Page 24]

RFC 2960 Stream Control Transmission Protocol October 2000
Fixed Parameters Status
----------------------------------------------
Initiate Tag Mandatory
Advertised Receiver Window Credit Mandatory
Number of Outbound Streams Mandatory
Number of Inbound Streams Mandatory
Initial TSN Mandatory
Variable Parameters Status Type Value
-------------------------------------------------------------
IPv4 Address (Note 1) Optional 5
IPv6 Address (Note 1) Optional 6
Cookie Preservative Optional 9
Reserved for ECN Capable (Note 2) Optional 32768 (0x8000)
Host Name Address (Note 3) Optional 11
Supported Address Types (Note 4) Optional 12
Note 1: The INIT chunks can contain multiple addresses that can be
IPv4 and/or IPv6 in any combination.
Note 2: The ECN capable field is reserved for future use of Explicit
Congestion Notification.
Note 3: An INIT chunk MUST NOT contain more than one Host Name
address parameter. Moreover, the sender of the INIT MUST NOT combine
any other address types with the Host Name address in the INIT. The
receiver of INIT MUST ignore any other address types if the Host Name
address parameter is present in the received INIT chunk.
Note 4: This parameter, when present, specifies all the address types
the sending endpoint can support. The absence of this parameter
indicates that the sending endpoint can support any address type.
The Chunk Flags field in INIT is reserved and all bits in it should
be set to 0 by the sender and ignored by the receiver. The sequence
of parameters within an INIT can be processed in any order.
Initiate Tag: 32 bits (unsigned integer)
The receiver of the INIT (the responding end) records the value of
the Initiate Tag parameter. This value MUST be placed into the
Verification Tag field of every SCTP packet that the receiver of
the INIT transmits within this association.
The Initiate Tag is allowed to have any value except 0. See
Section 5.3.1 for more on the selection of the tag value.
Stewart, et al. Standards Track [Page 25]

RFC 2960 Stream Control Transmission Protocol October 2000
If the value of the Initiate Tag in a received INIT chunk is found
to be 0, the receiver MUST treat it as an error and close the
association by transmitting an ABORT.
Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
integer)
This value represents the dedicated buffer space, in number of
bytes, the sender of the INIT has reserved in association with
this window. During the life of the association this buffer space
SHOULD not be lessened (i.e. dedicated buffers taken away from
this association); however, an endpoint MAY change the value of
a_rwnd it sends in SACK chunks.
Number of Outbound Streams (OS): 16 bits (unsigned integer)
Defines the number of outbound streams the sender of this INIT
chunk wishes to create in this association. The value of 0 MUST
NOT be used.
Note: A receiver of an INIT with the OS value set to 0 SHOULD
abort the association.
Number of Inbound Streams (MIS) : 16 bits (unsigned integer)
Defines the maximum number of streams the sender of this INIT
chunk allows the peer end to create in this association. The
value 0 MUST NOT be used.
Note: There is no negotiation of the actual number of streams but
instead the two endpoints will use the min(requested, offered).
See Section 5.1.1 for details.
Note: A receiver of an INIT with the MIS value of 0 SHOULD abort
the association.
Initial TSN (I-TSN) : 32 bits (unsigned integer)
Defines the initial TSN that the sender will use. The valid range
is from 0 to 4294967295. This field MAY be set to the value of
the Initiate Tag field.
3.3.2.1 Optional/Variable Length Parameters in INIT
The following parameters follow the Type-Length-Value format as
defined in Section 3.2.1. Any Type-Length-Value fields MUST come
after the fixed-length fields defined in the previous section.
Stewart, et al. Standards Track [Page 26]

RFC 2960 Stream Control Transmission Protocol October 2000
More than one IP Address parameter can be included in an INIT
chunk when the INIT sender is multi-homed. Moreover, a multi-
homed endpoint may have access to different types of network, thus
more than one address type can be present in one INIT chunk, i.e.,
IPv4 and IPv6 addresses are allowed in the same INIT chunk.
If the INIT contains at least one IP Address parameter, then the
source address of the IP datagram containing the INIT chunk and
any additional address(es) provided within the INIT can be used as
destinations by the endpoint receiving the INIT. If the INIT does
not contain any IP Address parameters, the endpoint receiving the
INIT MUST use the source address associated with the received IP
datagram as its sole destination address for the association.
Note that not using any IP address parameters in the INIT and
INIT-ACK is an alternative to make an association more likely to
work across a NAT box.
Cookie Preservative (9)
The sender of the INIT shall use this parameter to suggest to the
receiver of the INIT for a longer life-span of the State Cookie.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 9 | Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Suggested Cookie Life-span Increment (msec.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Suggested Cookie Life-span Increment: 32 bits (unsigned integer)
This parameter indicates to the receiver how much increment in
milliseconds the sender wishes the receiver to add to its default
cookie life-span.
This optional parameter should be added to the INIT chunk by the
sender when it re-attempts establishing an association with a peer
to which its previous attempt of establishing the association failed
due to a stale cookie operation error. The receiver MAY choose to
ignore the suggested cookie life-span increase for its own security
reasons.
Stewart, et al. Standards Track [Page 28]

RFC 2960 Stream Control Transmission Protocol October 20003.3.3 Initiation Acknowledgement (INIT ACK) (2):
The INIT ACK chunk is used to acknowledge the initiation of an SCTP
association.
The parameter part of INIT ACK is formatted similarly to the INIT
chunk. It uses two extra variable parameters: The State Cookie and
the Unrecognized Parameter:
The format of the INIT ACK chunk is shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 | Chunk Flags | Chunk Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiate Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Receiver Window Credit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Outbound Streams | Number of Inbound Streams |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initial TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ Optional/Variable-Length Parameters /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Initiate Tag: 32 bits (unsigned integer)
The receiver of the INIT ACK records the value of the Initiate Tag
parameter. This value MUST be placed into the Verification Tag
field of every SCTP packet that the INIT ACK receiver transmits
within this association.
The Initiate Tag MUST NOT take the value 0. See Section 5.3.1 for
more on the selection of the Initiate Tag value.
If the value of the Initiate Tag in a received INIT ACK chunk is
found to be 0, the receiver MUST treat it as an error and close
the association by transmitting an ABORT.
Stewart, et al. Standards Track [Page 30]

RFC 2960 Stream Control Transmission Protocol October 2000
Advertised Receiver Window Credit (a_rwnd): 32 bits (unsigned
integer)
This value represents the dedicated buffer space, in number of
bytes, the sender of the INIT ACK has reserved in association with
this window. During the life of the association this buffer space
SHOULD not be lessened (i.e. dedicated buffers taken away from
this association).
Number of Outbound Streams (OS): 16 bits (unsigned integer)
Defines the number of outbound streams the sender of this INIT ACK
chunk wishes to create in this association. The value of 0 MUST
NOT be used.
Note: A receiver of an INIT ACK with the OS value set to 0 SHOULD
destroy the association discarding its TCB.
Number of Inbound Streams (MIS) : 16 bits (unsigned integer)
Defines the maximum number of streams the sender of this INIT ACK
chunk allows the peer end to create in this association. The
value 0 MUST NOT be used.
Note: There is no negotiation of the actual number of streams but
instead the two endpoints will use the min(requested, offered).
See Section 5.1.1 for details.
Note: A receiver of an INIT ACK with the MIS value set to 0
SHOULD destroy the association discarding its TCB.
Initial TSN (I-TSN) : 32 bits (unsigned integer)
Defines the initial TSN that the INIT-ACK sender will use. The
valid range is from 0 to 4294967295. This field MAY be set to the
value of the Initiate Tag field.
Fixed Parameters Status
----------------------------------------------
Initiate Tag Mandatory
Advertised Receiver Window Credit Mandatory
Number of Outbound Streams Mandatory
Number of Inbound Streams Mandatory
Initial TSN Mandatory
Stewart, et al. Standards Track [Page 31]

RFC 2960 Stream Control Transmission Protocol October 2000
Variable Parameters Status Type Value
-------------------------------------------------------------
State Cookie Mandatory 7
IPv4 Address (Note 1) Optional 5
IPv6 Address (Note 1) Optional 6
Unrecognized Parameters Optional 8
Reserved for ECN Capable (Note 2) Optional 32768 (0x8000)
Host Name Address (Note 3) Optional 11
Note 1: The INIT ACK chunks can contain any number of IP address
parameters that can be IPv4 and/or IPv6 in any combination.
Note 2: The ECN capable field is reserved for future use of Explicit
Congestion Notification.
Note 3: The INIT ACK chunks MUST NOT contain more than one Host Name
address parameter. Moreover, the sender of the INIT ACK MUST NOT
combine any other address types with the Host Name address in the
INIT ACK. The receiver of the INIT ACK MUST ignore any other address
types if the Host Name address parameter is present.
IMPLEMENTATION NOTE: An implementation MUST be prepared to receive a
INIT ACK that is quite large (more than 1500 bytes) due to the
variable size of the state cookie AND the variable address list. For
example if a responder to the INIT has 1000 IPv4 addresses it wishes
to send, it would need at least 8,000 bytes to encode this in the
INIT ACK.
In combination with the Source Port carried in the SCTP common
header, each IP Address parameter in the INIT ACK indicates to the
receiver of the INIT ACK a valid transport address supported by the
sender of the INIT ACK for the lifetime of the association being
initiated.
If the INIT ACK contains at least one IP Address parameter, then the
source address of the IP datagram containing the INIT ACK and any
additional address(es) provided within the INIT ACK may be used as
destinations by the receiver of the INIT-ACK. If the INIT ACK does
not contain any IP Address parameters, the receiver of the INIT-ACK
MUST use the source address associated with the received IP datagram
as its sole destination address for the association.
The State Cookie and Unrecognized Parameters use the Type-Length-
Value format as defined in Section 3.2.1 and are described below.
The other fields are defined the same as their counterparts in the
INIT chunk.
Stewart, et al. Standards Track [Page 32]

RFC 2960 Stream Control Transmission Protocol October 20003.3.3.1 Optional or Variable Length Parameters
State Cookie
Parameter Type Value: 7
Parameter Length: variable size, depending on Size of Cookie
Parameter Value:
This parameter value MUST contain all the necessary state and
parameter information required for the sender of this INIT ACK
to create the association, along with a Message Authentication
Code (MAC). See Section 5.1.3 for details on State Cookie
definition.
Unrecognized Parameters:
Parameter Type Value: 8
Parameter Length: Variable Size.
Parameter Value:
This parameter is returned to the originator of the INIT chunk
when the INIT contains an unrecognized parameter which has a
value that indicates that it should be reported to the sender.
This parameter value field will contain unrecognized parameters
copied from the INIT chunk complete with Parameter Type, Length
and Value fields.
3.3.4 Selective Acknowledgement (SACK) (3):
This chunk is sent to the peer endpoint to acknowledge received DATA
chunks and to inform the peer endpoint of gaps in the received
subsequences of DATA chunks as represented by their TSNs.
The SACK MUST contain the Cumulative TSN Ack and Advertised Receiver
Window Credit (a_rwnd) parameters.
By definition, the value of the Cumulative TSN Ack parameter is the
last TSN received before a break in the sequence of received TSNs
occurs; the next TSN value following this one has not yet been
received at the endpoint sending the SACK. This parameter therefore
acknowledges receipt of all TSNs less than or equal to its value.
The handling of a_rwnd by the receiver of the SACK is discussed in
detail in Section 6.2.1.
Stewart, et al. Standards Track [Page 33]

RFC 2960 Stream Control Transmission Protocol October 2000
Number of Gap Ack Blocks: 16 bits (unsigned integer)
Indicates the number of Gap Ack Blocks included in this SACK.
Number of Duplicate TSNs: 16 bit
This field contains the number of duplicate TSNs the endpoint has
received. Each duplicate TSN is listed following the Gap Ack
Block list.
Gap Ack Blocks:
These fields contain the Gap Ack Blocks. They are repeated for
each Gap Ack Block up to the number of Gap Ack Blocks defined in
the Number of Gap Ack Blocks field. All DATA chunks with TSNs
greater than or equal to (Cumulative TSN Ack + Gap Ack Block
Start) and less than or equal to (Cumulative TSN Ack + Gap Ack
Block End) of each Gap Ack Block are assumed to have been received
correctly.
Gap Ack Block Start: 16 bits (unsigned integer)
Indicates the Start offset TSN for this Gap Ack Block. To
calculate the actual TSN number the Cumulative TSN Ack is added to
this offset number. This calculated TSN identifies the first TSN
in this Gap Ack Block that has been received.
Gap Ack Block End: 16 bits (unsigned integer)
Indicates the End offset TSN for this Gap Ack Block. To calculate
the actual TSN number the Cumulative TSN Ack is added to this
offset number. This calculated TSN identifies the TSN of the last
DATA chunk received in this Gap Ack Block.
For example, assume the receiver has the following DATA chunks newly
arrived at the time when it decides to send a Selective ACK,
Stewart, et al. Standards Track [Page 35]

RFC 2960 Stream Control Transmission Protocol October 2000
----------
| TSN=17 |
----------
| | <- still missing
----------
| TSN=15 |
----------
| TSN=14 |
----------
| | <- still missing
----------
| TSN=12 |
----------
| TSN=11 |
----------
| TSN=10 |
----------
then, the parameter part of the SACK MUST be constructed as follows
(assuming the new a_rwnd is set to 4660 by the sender):
+--------------------------------+
| Cumulative TSN Ack = 12 |
+--------------------------------+
| a_rwnd = 4660 |
+----------------+---------------+
| num of block=2 | num of dup=0 |
+----------------+---------------+
|block #1 strt=2 |block #1 end=3 |
+----------------+---------------+
|block #2 strt=5 |block #2 end=5 |
+----------------+---------------+
Duplicate TSN: 32 bits (unsigned integer)
Indicates the number of times a TSN was received in duplicate
since the last SACK was sent. Every time a receiver gets a
duplicate TSN (before sending the SACK) it adds it to the list of
duplicates. The duplicate count is re-initialized to zero after
sending each SACK.
For example, if a receiver were to get the TSN 19 three times it
would list 19 twice in the outbound SACK. After sending the SACK
if it received yet one more TSN 19 it would list 19 as a duplicate
once in the next outgoing SACK.
Stewart, et al. Standards Track [Page 36]

RFC 2960 Stream Control Transmission Protocol October 20003.3.7 Abort Association (ABORT) (6):
The ABORT chunk is sent to the peer of an association to close the
association. The ABORT chunk may contain Cause Parameters to inform
the receiver the reason of the abort. DATA chunks MUST NOT be
bundled with ABORT. Control chunks (except for INIT, INIT ACK and
SHUTDOWN COMPLETE) MAY be bundled with an ABORT but they MUST be
placed before the ABORT in the SCTP packet, or they will be ignored
by the receiver.
If an endpoint receives an ABORT with a format error or for an
association that doesn't exist, it MUST silently discard it.
Moreover, under any circumstances, an endpoint that receives an ABORT
MUST NOT respond to that ABORT by sending an ABORT of its own.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 6 |Reserved |T| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ zero or more Error Causes /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Flags: 8 bits
Reserved: 7 bits
Set to 0 on transmit and ignored on receipt.
T bit: 1 bit
The T bit is set to 0 if the sender had a TCB that it destroyed.
If the sender did not have a TCB it should set this bit to 1.
Note: Special rules apply to this chunk for verification, please see
Section 8.5.1 for details.
Length: 16 bits (unsigned integer)
Set to the size of the chunk in bytes, including the chunk header
and all the Error Cause fields present.
See Section 3.3.10 for Error Cause definitions.
Stewart, et al. Standards Track [Page 39]

RFC 2960 Stream Control Transmission Protocol October 2000
The sender of this error cause MAY choose to report how long past
expiration the State Cookie is by including a non-zero value in
the Measure of Staleness field. If the sender does not wish to
provide this information it should set the Measure of Staleness
field to the value of zero.
3.3.10.4 Out of Resource (4)
Cause of error
---------------
Out of Resource: Indicates that the sender is out of resource. This
is usually sent in combination with or within an ABORT.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=4 | Cause Length=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.3.10.5 Unresolvable Address (5)
Cause of error
---------------
Unresolvable Address: Indicates that the sender is not able to
resolve the specified address parameter (e.g., type of address is not
supported by the sender). This is usually sent in combination with
or within an ABORT.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=5 | Cause Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Unresolvable Address /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Unresolvable Address: variable length
The unresolvable address field contains the complete Type, Length
and Value of the address parameter (or Host Name parameter) that
contains the unresolvable address or host name.
3.3.10.6 Unrecognized Chunk Type (6)
Cause of error
---------------
Unrecognized Chunk Type: This error cause is returned to the
originator of the chunk if the receiver does not understand the chunk
and the upper bits of the 'Chunk Type' are set to 01 or 11.
Stewart, et al. Standards Track [Page 44]

RFC 2960 Stream Control Transmission Protocol October 20003.3.10.9 No User Data (9)
Cause of error
---------------
No User Data: This error cause is returned to the originator of a
DATA chunk if a received DATA chunk has no user data.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=9 | Cause Length=8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ TSN value /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TSN value: 32 bits (+unsigned integer)
The TSN value field contains the TSN of the DATA chunk received
with no user data field.
This cause code is normally returned in an ABORT chunk (see
Section 6.2)
3.3.10.10 Cookie Received While Shutting Down (10)
Cause of error
---------------
Cookie Received While Shutting Down: A COOKIE ECHO was received
While the endpoint was in SHUTDOWN-ACK-SENT state. This error is
usually returned in an ERROR chunk bundled with the retransmitted
SHUTDOWN ACK.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Cause Code=10 | Cause Length=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.3.11 Cookie Echo (COOKIE ECHO) (10):
This chunk is used only during the initialization of an association.
It is sent by the initiator of an association to its peer to complete
the initialization process. This chunk MUST precede any DATA chunk
sent within the association, but MAY be bundled with one or more DATA
chunks in the same packet.
Stewart, et al. Standards Track [Page 46]

RFC 2960 Stream Control Transmission Protocol October 20003.3.13 Shutdown Complete (SHUTDOWN COMPLETE) (14):
This chunk MUST be used to acknowledge the receipt of the SHUTDOWN
ACK chunk at the completion of the shutdown process, see Section 9.2
for details.
The SHUTDOWN COMPLETE chunk has no parameters.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 14 |Reserved |T| Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Chunk Flags: 8 bits
Reserved: 7 bits
Set to 0 on transmit and ignored on receipt.
T bit: 1 bit
The T bit is set to 0 if the sender had a TCB that it destroyed.
If the sender did not have a TCB it should set this bit to 1.
Note: Special rules apply to this chunk for verification, please see
Section 8.5.1 for details.
4. SCTP Association State Diagram
During the lifetime of an SCTP association, the SCTP endpoint's
association progress from one state to another in response to various
events. The events that may potentially advance an association's
state include:
o SCTP user primitive calls, e.g., [ASSOCIATE], [SHUTDOWN], [ABORT],
o Reception of INIT, COOKIE ECHO, ABORT, SHUTDOWN, etc., control
chunks, or
o Some timeout events.
The state diagram in the figures below illustrates state changes,
together with the causing events and resulting actions. Note that
some of the error conditions are not shown in the state diagram.
Full description of all special cases should be found in the text.
Stewart, et al. Standards Track [Page 48]

RFC 2960 Stream Control Transmission Protocol October 2000
| | (D)rcv SHUTDOWN ACK
| |--------------
| | stop shutdown timer
| | send SHUTDOWN COMPLETE
| | delete TCB
| |
\ +---------+ /
\-->| CLOSED |<--/
+---------+
Figure 3: State Transition Diagram of SCTP
Notes:
1) If the State Cookie in the received COOKIE ECHO is invalid (i.e.,
failed to pass the integrity check), the receiver MUST silently
discard the packet. Or, if the received State Cookie is expired
(see Section 5.1.5), the receiver MUST send back an ERROR chunk.
In either case, the receiver stays in the CLOSED state.
2) If the T1-init timer expires, the endpoint MUST retransmit INIT
and re-start the T1-init timer without changing state. This MUST
be repeated up to 'Max.Init.Retransmits' times. After that, the
endpoint MUST abort the initialization process and report the
error to SCTP user.
3) If the T1-cookie timer expires, the endpoint MUST retransmit
COOKIE ECHO and re-start the T1-cookie timer without changing
state. This MUST be repeated up to 'Max.Init.Retransmits' times.
After that, the endpoint MUST abort the initialization process and
report the error to SCTP user.
4) In SHUTDOWN-SENT state the endpoint MUST acknowledge any received
DATA chunks without delay.
5) In SHUTDOWN-RECEIVED state, the endpoint MUST NOT accept any new
send request from its SCTP user.
6) In SHUTDOWN-RECEIVED state, the endpoint MUST transmit or
retransmit data and leave this state when all data in queue is
transmitted.
7) In SHUTDOWN-ACK-SENT state, the endpoint MUST NOT accept any new
send request from its SCTP user.
The CLOSED state is used to indicate that an association is not
created (i.e., doesn't exist).
Stewart, et al. Standards Track [Page 51]

RFC 2960 Stream Control Transmission Protocol October 20005. Association Initialization
Before the first data transmission can take place from one SCTP
endpoint ("A") to another SCTP endpoint ("Z"), the two endpoints must
complete an initialization process in order to set up an SCTP
association between them.
The SCTP user at an endpoint should use the ASSOCIATE primitive to
initialize an SCTP association to another SCTP endpoint.
IMPLEMENTATION NOTE: From an SCTP-user's point of view, an
association may be implicitly opened, without an ASSOCIATE primitive
(see 10.1 B) being invoked, by the initiating endpoint's sending of
the first user data to the destination endpoint. The initiating SCTP
will assume default values for all mandatory and optional parameters
for the INIT/INIT ACK.
Once the association is established, unidirectional streams are open
for data transfer on both ends (see Section 5.1.1).
5.1 Normal Establishment of an Association
The initialization process consists of the following steps (assuming
that SCTP endpoint "A" tries to set up an association with SCTP
endpoint "Z" and "Z" accepts the new association):
A) "A" first sends an INIT chunk to "Z". In the INIT, "A" must
provide its Verification Tag (Tag_A) in the Initiate Tag field.
Tag_A SHOULD be a random number in the range of 1 to 4294967295
(see 5.3.1 for Tag value selection). After sending the INIT, "A"
starts the T1-init timer and enters the COOKIE-WAIT state.
B) "Z" shall respond immediately with an INIT ACK chunk. The
destination IP address of the INIT ACK MUST be set to the source
IP address of the INIT to which this INIT ACK is responding. In
the response, besides filling in other parameters, "Z" must set
the Verification Tag field to Tag_A, and also provide its own
Verification Tag (Tag_Z) in the Initiate Tag field.
Moreover, "Z" MUST generate and send along with the INIT ACK a
State Cookie. See Section 5.1.3 for State Cookie generation.
Note: After sending out INIT ACK with the State Cookie parameter,
"Z" MUST NOT allocate any resources, nor keep any states for the
new association. Otherwise, "Z" will be vulnerable to resource
attacks.
Stewart, et al. Standards Track [Page 52]

RFC 2960 Stream Control Transmission Protocol October 2000
C) Upon reception of the INIT ACK from "Z", "A" shall stop the T1-
init timer and leave COOKIE-WAIT state. "A" shall then send the
State Cookie received in the INIT ACK chunk in a COOKIE ECHO
chunk, start the T1-cookie timer, and enter the COOKIE-ECHOED
state.
Note: The COOKIE ECHO chunk can be bundled with any pending
outbound DATA chunks, but it MUST be the first chunk in the packet
and until the COOKIE ACK is returned the sender MUST NOT send any
other packets to the peer.
D) Upon reception of the COOKIE ECHO chunk, Endpoint "Z" will reply
with a COOKIE ACK chunk after building a TCB and moving to the
ESTABLISHED state. A COOKIE ACK chunk may be bundled with any
pending DATA chunks (and/or SACK chunks), but the COOKIE ACK chunk
MUST be the first chunk in the packet.
IMPLEMENTATION NOTE: An implementation may choose to send the
Communication Up notification to the SCTP user upon reception of a
valid COOKIE ECHO chunk.
E) Upon reception of the COOKIE ACK, endpoint "A" will move from the
COOKIE-ECHOED state to the ESTABLISHED state, stopping the T1-
cookie timer. It may also notify its ULP about the successful
establishment of the association with a Communication Up
notification (see Section 10).
An INIT or INIT ACK chunk MUST NOT be bundled with any other chunk.
They MUST be the only chunks present in the SCTP packets that carry
them.
An endpoint MUST send the INIT ACK to the IP address from which it
received the INIT.
Note: T1-init timer and T1-cookie timer shall follow the same rules
given in Section 6.3.
If an endpoint receives an INIT, INIT ACK, or COOKIE ECHO chunk but
decides not to establish the new association due to missing mandatory
parameters in the received INIT or INIT ACK, invalid parameter
values, or lack of local resources, it MUST respond with an ABORT
chunk. It SHOULD also specify the cause of abort, such as the type
of the missing mandatory parameters, etc., by including the error
cause parameters with the ABORT chunk. The Verification Tag field in
the common header of the outbound SCTP packet containing the ABORT
chunk MUST be set to the Initiate Tag value of the peer.
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RFC 2960 Stream Control Transmission Protocol October 2000
After the reception of the first DATA chunk in an association the
endpoint MUST immediately respond with a SACK to acknowledge the DATA
chunk. Subsequent acknowledgements should be done as described in
Section 6.2.
When the TCB is created, each endpoint MUST set its internal
Cumulative TSN Ack Point to the value of its transmitted Initial TSN
minus one.
IMPLEMENTATION NOTE: The IP addresses and SCTP port are generally
used as the key to find the TCB within an SCTP instance.
5.1.1 Handle Stream Parameters
In the INIT and INIT ACK chunks, the sender of the chunk shall
indicate the number of outbound streams (OS) it wishes to have in the
association, as well as the maximum inbound streams (MIS) it will
accept from the other endpoint.
After receiving the stream configuration information from the other
side, each endpoint shall perform the following check: If the peer's
MIS is less than the endpoint's OS, meaning that the peer is
incapable of supporting all the outbound streams the endpoint wants
to configure, the endpoint MUST either use MIS outbound streams, or
abort the association and report to its upper layer the resources
shortage at its peer.
After the association is initialized, the valid outbound stream
identifier range for either endpoint shall be 0 to min(local OS,
remote MIS)-1.
5.1.2 Handle Address Parameters
During the association initialization, an endpoint shall use the
following rules to discover and collect the destination transport
address(es) of its peer.
A) If there are no address parameters present in the received INIT or
INIT ACK chunk, the endpoint shall take the source IP address from
which the chunk arrives and record it, in combination with the
SCTP source port number, as the only destination transport address
for this peer.
B) If there is a Host Name parameter present in the received INIT or
INIT ACK chunk, the endpoint shall resolve that host name to a
list of IP address(es) and derive the transport address(es) of
this peer by combining the resolved IP address(es) with the SCTP
source port.
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RFC 2960 Stream Control Transmission Protocol October 2000
The endpoint MUST ignore any other IP address parameters if they
are also present in the received INIT or INIT ACK chunk.
The time at which the receiver of an INIT resolves the host name
has potential security implications to SCTP. If the receiver of
an INIT resolves the host name upon the reception of the chunk,
and the mechanism the receiver uses to resolve the host name
involves potential long delay (e.g. DNS query), the receiver may
open itself up to resource attacks for the period of time while it
is waiting for the name resolution results before it can build the
State Cookie and release local resources.
Therefore, in cases where the name translation involves potential
long delay, the receiver of the INIT MUST postpone the name
resolution till the reception of the COOKIE ECHO chunk from the
peer. In such a case, the receiver of the INIT SHOULD build the
State Cookie using the received Host Name (instead of destination
transport addresses) and send the INIT ACK to the source IP
address from which the INIT was received.
The receiver of an INIT ACK shall always immediately attempt to
resolve the name upon the reception of the chunk.
The receiver of the INIT or INIT ACK MUST NOT send user data
(piggy-backed or stand-alone) to its peer until the host name is
successfully resolved.
If the name resolution is not successful, the endpoint MUST
immediately send an ABORT with "Unresolvable Address" error cause
to its peer. The ABORT shall be sent to the source IP address
from which the last peer packet was received.
C) If there are only IPv4/IPv6 addresses present in the received INIT
or INIT ACK chunk, the receiver shall derive and record all the
transport address(es) from the received chunk AND the source IP
address that sent the INIT or INIT ACK. The transport address(es)
are derived by the combination of SCTP source port (from the
common header) and the IP address parameter(s) carried in the INIT
or INIT ACK chunk and the source IP address of the IP datagram.
The receiver should use only these transport addresses as
destination transport addresses when sending subsequent packets to
its peer.
IMPLEMENTATION NOTE: In some cases (e.g., when the implementation
doesn't control the source IP address that is used for
transmitting), an endpoint might need to include in its INIT or
INIT ACK all possible IP addresses from which packets to the peer
could be transmitted.
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RFC 2960 Stream Control Transmission Protocol October 2000
After all transport addresses are derived from the INIT or INIT ACK
chunk using the above rules, the endpoint shall select one of the
transport addresses as the initial primary path.
Note: The INIT-ACK MUST be sent to the source address of the INIT.
The sender of INIT may include a 'Supported Address Types' parameter
in the INIT to indicate what types of address are acceptable. When
this parameter is present, the receiver of INIT (initiatee) MUST
either use one of the address types indicated in the Supported
Address Types parameter when responding to the INIT, or abort the
association with an "Unresolvable Address" error cause if it is
unwilling or incapable of using any of the address types indicated by
its peer.
IMPLEMENTATION NOTE: In the case that the receiver of an INIT ACK
fails to resolve the address parameter due to an unsupported type, it
can abort the initiation process and then attempt a re-initiation by
using a 'Supported Address Types' parameter in the new INIT to
indicate what types of address it prefers.
5.1.3 Generating State Cookie
When sending an INIT ACK as a response to an INIT chunk, the sender
of INIT ACK creates a State Cookie and sends it in the State Cookie
parameter of the INIT ACK. Inside this State Cookie, the sender
should include a MAC (see [RFC2104] for an example), a time stamp on
when the State Cookie is created, and the lifespan of the State
Cookie, along with all the information necessary for it to establish
the association.
The following steps SHOULD be taken to generate the State Cookie:
1) Create an association TCB using information from both the received
INIT and the outgoing INIT ACK chunk,
2) In the TCB, set the creation time to the current time of day, and
the lifespan to the protocol parameter 'Valid.Cookie.Life',
3) From the TCB, identify and collect the minimal subset of
information needed to re-create the TCB, and generate a MAC using
this subset of information and a secret key (see [RFC2104] for an
example of generating a MAC), and
4) Generate the State Cookie by combining this subset of information
and the resultant MAC.
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RFC 2960 Stream Control Transmission Protocol October 2000
After sending the INIT ACK with the State Cookie parameter, the
sender SHOULD delete the TCB and any other local resource related to
the new association, so as to prevent resource attacks.
The hashing method used to generate the MAC is strictly a private
matter for the receiver of the INIT chunk. The use of a MAC is
mandatory to prevent denial of service attacks. The secret key
SHOULD be random ([RFC1750] provides some information on randomness
guidelines); it SHOULD be changed reasonably frequently, and the
timestamp in the State Cookie MAY be used to determine which key
should be used to verify the MAC.
An implementation SHOULD make the cookie as small as possible to
insure interoperability.
5.1.4 State Cookie Processing
When an endpoint (in the COOKIE WAIT state) receives an INIT ACK
chunk with a State Cookie parameter, it MUST immediately send a
COOKIE ECHO chunk to its peer with the received State Cookie. The
sender MAY also add any pending DATA chunks to the packet after the
COOKIE ECHO chunk.
The endpoint shall also start the T1-cookie timer after sending out
the COOKIE ECHO chunk. If the timer expires, the endpoint shall
retransmit the COOKIE ECHO chunk and restart the T1-cookie timer.
This is repeated until either a COOKIE ACK is received or '
Max.Init.Retransmits' is reached causing the peer endpoint to be
marked unreachable (and thus the association enters the CLOSED
state).
5.1.5 State Cookie Authentication
When an endpoint receives a COOKIE ECHO chunk from another endpoint
with which it has no association, it shall take the following
actions:
1) Compute a MAC using the TCB data carried in the State Cookie and
the secret key (note the timestamp in the State Cookie MAY be used
to determine which secret key to use). Reference [RFC2104] can be
used as a guideline for generating the MAC,
2) Authenticate the State Cookie as one that it previously generated
by comparing the computed MAC against the one carried in the State
Cookie. If this comparison fails, the SCTP packet, including the
COOKIE ECHO and any DATA chunks, should be silently discarded,
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RFC 2960 Stream Control Transmission Protocol October 2000
3) Compare the creation timestamp in the State Cookie to the current
local time. If the elapsed time is longer than the lifespan
carried in the State Cookie, then the packet, including the COOKIE
ECHO and any attached DATA chunks, SHOULD be discarded and the
endpoint MUST transmit an ERROR chunk with a "Stale Cookie" error
cause to the peer endpoint,
4) If the State Cookie is valid, create an association to the sender
of the COOKIE ECHO chunk with the information in the TCB data
carried in the COOKIE ECHO, and enter the ESTABLISHED state,
5) Send a COOKIE ACK chunk to the peer acknowledging reception of the
COOKIE ECHO. The COOKIE ACK MAY be bundled with an outbound DATA
chunk or SACK chunk; however, the COOKIE ACK MUST be the first
chunk in the SCTP packet.
6) Immediately acknowledge any DATA chunk bundled with the COOKIE
ECHO with a SACK (subsequent DATA chunk acknowledgement should
follow the rules defined in Section 6.2). As mentioned in step
5), if the SACK is bundled with the COOKIE ACK, the COOKIE ACK
MUST appear first in the SCTP packet.
If a COOKIE ECHO is received from an endpoint with which the receiver
of the COOKIE ECHO has an existing association, the procedures in
Section 5.2 should be followed.
5.1.6 An Example of Normal Association Establishment
In the following example, "A" initiates the association and then
sends a user message to "Z", then "Z" sends two user messages to "A"
later (assuming no bundling or fragmentation occurs):
Stewart, et al. Standards Track [Page 58]

RFC 2960 Stream Control Transmission Protocol October 2000
the timer restarted. This shall be repeated Max.Init.Retransmits
times before "A" considers "Z" unreachable and reports the failure to
its upper layer (and thus the association enters the CLOSED state).
When retransmitting the INIT, the endpoint MUST follow the rules
defined in 6.3 to determine the proper timer value.
5.2 Handle Duplicate or Unexpected INIT, INIT ACK, COOKIE ECHO, and COOKIE ACK
During the lifetime of an association (in one of the possible
states), an endpoint may receive from its peer endpoint one of the
setup chunks (INIT, INIT ACK, COOKIE ECHO, and COOKIE ACK). The
receiver shall treat such a setup chunk as a duplicate and process it
as described in this section.
Note: An endpoint will not receive the chunk unless the chunk was
sent to a SCTP transport address and is from a SCTP transport address
associated with this endpoint. Therefore, the endpoint processes
such a chunk as part of its current association.
The following scenarios can cause duplicated or unexpected chunks:
A) The peer has crashed without being detected, re-started itself and
sent out a new INIT chunk trying to restore the association,
B) Both sides are trying to initialize the association at about the
same time,
C) The chunk is from a stale packet that was used to establish the
present association or a past association that is no longer in
existence,
D) The chunk is a false packet generated by an attacker, or
E) The peer never received the COOKIE ACK and is retransmitting its
COOKIE ECHO.
The rules in the following sections shall be applied in order to
identify and correctly handle these cases.
5.2.1 INIT received in COOKIE-WAIT or COOKIE-ECHOED State (Item B)
This usually indicates an initialization collision, i.e., each
endpoint is attempting, at about the same time, to establish an
association with the other endpoint.
Upon receipt of an INIT in the COOKIE-WAIT or COOKIE-ECHOED state, an
endpoint MUST respond with an INIT ACK using the same parameters it
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RFC 2960 Stream Control Transmission Protocol October 2000
sent in its original INIT chunk (including its Initiation Tag,
unchanged). These original parameters are combined with those from
the newly received INIT chunk. The endpoint shall also generate a
State Cookie with the INIT ACK. The endpoint uses the parameters
sent in its INIT to calculate the State Cookie.
After that, the endpoint MUST NOT change its state, the T1-init timer
shall be left running and the corresponding TCB MUST NOT be
destroyed. The normal procedures for handling State Cookies when a
TCB exists will resolve the duplicate INITs to a single association.
For an endpoint that is in the COOKIE-ECHOED state it MUST populate
its Tie-Tags with the Tag information of itself and its peer (see
section 5.2.2 for a description of the Tie-Tags).
5.2.2 Unexpected INIT in States Other than CLOSED, COOKIE-ECHOED, COOKIE-WAIT and SHUTDOWN-ACK-SENT
Unless otherwise stated, upon reception of an unexpected INIT for
this association, the endpoint shall generate an INIT ACK with a
State Cookie. In the outbound INIT ACK the endpoint MUST copy its
current Verification Tag and peer's Verification Tag into a reserved
place within the state cookie. We shall refer to these locations as
the Peer's-Tie-Tag and the Local-Tie-Tag. The outbound SCTP packet
containing this INIT ACK MUST carry a Verification Tag value equal to
the Initiation Tag found in the unexpected INIT. And the INIT ACK
MUST contain a new Initiation Tag (randomly generated see Section5.3.1). Other parameters for the endpoint SHOULD be copied from the
existing parameters of the association (e.g. number of outbound
streams) into the INIT ACK and cookie.
After sending out the INIT ACK, the endpoint shall take no further
actions, i.e., the existing association, including its current state,
and the corresponding TCB MUST NOT be changed.
Note: Only when a TCB exists and the association is not in a COOKIE-
WAIT state are the Tie-Tags populated. For a normal association INIT
(i.e. the endpoint is in a COOKIE-WAIT state), the Tie-Tags MUST be
set to 0 (indicating that no previous TCB existed). The INIT ACK and
State Cookie are populated as specified in section 5.2.1.
5.2.3 Unexpected INIT ACK
If an INIT ACK is received by an endpoint in any state other than the
COOKIE-WAIT state, the endpoint should discard the INIT ACK chunk.
An unexpected INIT ACK usually indicates the processing of an old or
duplicated INIT chunk.
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RFC 2960 Stream Control Transmission Protocol October 20005.2.4 Handle a COOKIE ECHO when a TCB exists
When a COOKIE ECHO chunk is received by an endpoint in any state for
an existing association (i.e., not in the CLOSED state) the following
rules shall be applied:
1) Compute a MAC as described in Step 1 of Section 5.1.5,
2) Authenticate the State Cookie as described in Step 2 of Section5.1.5 (this is case C or D above).
3) Compare the timestamp in the State Cookie to the current time. If
the State Cookie is older than the lifespan carried in the State
Cookie and the Verification Tags contained in the State Cookie do
not match the current association's Verification Tags, the packet,
including the COOKIE ECHO and any DATA chunks, should be
discarded. The endpoint also MUST transmit an ERROR chunk with a
"Stale Cookie" error cause to the peer endpoint (this is case C or
D in section 5.2).
If both Verification Tags in the State Cookie match the
Verification Tags of the current association, consider the State
Cookie valid (this is case E of section 5.2) even if the lifespan
is exceeded.
4) If the State Cookie proves to be valid, unpack the TCB into a
temporary TCB.
5) Refer to Table 2 to determine the correct action to be taken.
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RFC 2960 Stream Control Transmission Protocol October 2000
+------------+------------+---------------+--------------+-------------+
| Local Tag | Peer's Tag | Local-Tie-Tag |Peer's-Tie-Tag| Action/ |
| | | | | Description |
+------------+------------+---------------+--------------+-------------+
| X | X | M | M | (A) |
+------------+------------+---------------+--------------+-------------+
| M | X | A | A | (B) |
+------------+------------+---------------+--------------+-------------+
| M | 0 | A | A | (B) |
+------------+------------+---------------+--------------+-------------+
| X | M | 0 | 0 | (C) |
+------------+------------+---------------+--------------+-------------+
| M | M | A | A | (D) |
+======================================================================+
| Table 2: Handling of a COOKIE ECHO when a TCB exists |
+======================================================================+
Legend:
X - Tag does not match the existing TCB
M - Tag matches the existing TCB.
0 - No Tie-Tag in Cookie (unknown).
A - All cases, i.e. M, X or 0.
Note: For any case not shown in Table 2, the cookie should be
silently discarded.
Action
A) In this case, the peer may have restarted. When the endpoint
recognizes this potential 'restart', the existing session is
treated the same as if it received an ABORT followed by a new
COOKIE ECHO with the following exceptions:
- Any SCTP DATA Chunks MAY be retained (this is an implementation
specific option).
- A notification of RESTART SHOULD be sent to the ULP instead of
a "COMMUNICATION LOST" notification.
All the congestion control parameters (e.g., cwnd, ssthresh)
related to this peer MUST be reset to their initial values (see
Section 6.2.1).
After this the endpoint shall enter the ESTABLISHED state.
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RFC 2960 Stream Control Transmission Protocol October 2000
If the endpoint is in the SHUTDOWN-ACK-SENT state and recognizes
the peer has restarted (Action A), it MUST NOT setup a new
association but instead resend the SHUTDOWN ACK and send an ERROR
chunk with a "Cookie Received while Shutting Down" error cause to
its peer.
B) In this case, both sides may be attempting to start an association
at about the same time but the peer endpoint started its INIT
after responding to the local endpoint's INIT. Thus it may have
picked a new Verification Tag not being aware of the previous Tag
it had sent this endpoint. The endpoint should stay in or enter
the ESTABLISHED state but it MUST update its peer's Verification
Tag from the State Cookie, stop any init or cookie timers that may
running and send a COOKIE ACK.
C) In this case, the local endpoint's cookie has arrived late.
Before it arrived, the local endpoint sent an INIT and received an
INIT-ACK and finally sent a COOKIE ECHO with the peer's same tag
but a new tag of its own. The cookie should be silently
discarded. The endpoint SHOULD NOT change states and should leave
any timers running.
D) When both local and remote tags match the endpoint should always
enter the ESTABLISHED state, if it has not already done so. It
should stop any init or cookie timers that may be running and send
a COOKIE ACK.
Note: The "peer's Verification Tag" is the tag received in the
Initiate Tag field of the INIT or INIT ACK chunk.
5.2.4.1 An Example of a Association Restart
In the following example, "A" initiates the association after a
restart has occurred. Endpoint "Z" had no knowledge of the restart
until the exchange (i.e. Heartbeats had not yet detected the failure
of "A"). (assuming no bundling or fragmentation occurs):
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RFC 2960 Stream Control Transmission Protocol October 20005.2.5 Handle Duplicate COOKIE-ACK.
At any state other than COOKIE-ECHOED, an endpoint should silently
discard a received COOKIE ACK chunk.
5.2.6 Handle Stale COOKIE Error
Receipt of an ERROR chunk with a "Stale Cookie" error cause indicates
one of a number of possible events:
A) That the association failed to completely setup before the State
Cookie issued by the sender was processed.
B) An old State Cookie was processed after setup completed.
C) An old State Cookie is received from someone that the receiver is
not interested in having an association with and the ABORT chunk
was lost.
When processing an ERROR chunk with a "Stale Cookie" error cause an
endpoint should first examine if an association is in the process of
being setup, i.e. the association is in the COOKIE-ECHOED state. In
all cases if the association is not in the COOKIE-ECHOED state, the
ERROR chunk should be silently discarded.
If the association is in the COOKIE-ECHOED state, the endpoint may
elect one of the following three alternatives.
1) Send a new INIT chunk to the endpoint to generate a new State
Cookie and re-attempt the setup procedure.
2) Discard the TCB and report to the upper layer the inability to
setup the association.
3) Send a new INIT chunk to the endpoint, adding a Cookie
Preservative parameter requesting an extension to the lifetime of
the State Cookie. When calculating the time extension, an
implementation SHOULD use the RTT information measured based on
the previous COOKIE ECHO / ERROR exchange, and should add no more
than 1 second beyond the measured RTT, due to long State Cookie
lifetimes making the endpoint more subject to a replay attack.
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RFC 2960 Stream Control Transmission Protocol October 20005.3 Other Initialization Issues5.3.1 Selection of Tag Value
Initiate Tag values should be selected from the range of 1 to 2**32 -
1. It is very important that the Initiate Tag value be randomized to
help protect against "man in the middle" and "sequence number"
attacks. The methods described in [RFC1750] can be used for the
Initiate Tag randomization. Careful selection of Initiate Tags is
also necessary to prevent old duplicate packets from previous
associations being mistakenly processed as belonging to the current
association.
Moreover, the Verification Tag value used by either endpoint in a
given association MUST NOT change during the lifetime of an
association. A new Verification Tag value MUST be used each time the
endpoint tears-down and then re-establishes an association to the
same peer.
6. User Data Transfer
Data transmission MUST only happen in the ESTABLISHED, SHUTDOWN-
PENDING, and SHUTDOWN-RECEIVED states. The only exception to this is
that DATA chunks are allowed to be bundled with an outbound COOKIE
ECHO chunk when in COOKIE-WAIT state.
DATA chunks MUST only be received according to the rules below in
ESTABLISHED, SHUTDOWN-PENDING, SHUTDOWN-SENT. A DATA chunk received
in CLOSED is out of the blue and SHOULD be handled per 8.4. A DATA
chunk received in any other state SHOULD be discarded.
A SACK MUST be processed in ESTABLISHED, SHUTDOWN-PENDING, and
SHUTDOWN-RECEIVED. An incoming SACK MAY be processed in COOKIE-
ECHOED. A SACK in the CLOSED state is out of the blue and SHOULD be
processed according to the rules in 8.4. A SACK chunk received in
any other state SHOULD be discarded.
A SCTP receiver MUST be able to receive a minimum of 1500 bytes in
one SCTP packet. This means that a SCTP endpoint MUST NOT indicate
less than 1500 bytes in its Initial a_rwnd sent in the INIT or INIT
ACK.
For transmission efficiency, SCTP defines mechanisms for bundling of
small user messages and fragmentation of large user messages. The
following diagram depicts the flow of user messages through SCTP.
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RFC 2960 Stream Control Transmission Protocol October 2000
In this section the term "data sender" refers to the endpoint that
transmits a DATA chunk and the term "data receiver" refers to the
endpoint that receives a DATA chunk. A data receiver will transmit
SACK chunks.
+--------------------------+
| User Messages |
+--------------------------+
SCTP user ^ |
==================|==|=======================================
| v (1)
+------------------+ +--------------------+
| SCTP DATA Chunks | |SCTP Control Chunks |
+------------------+ +--------------------+
^ | ^ |
| v (2) | v (2)
+--------------------------+
| SCTP packets |
+--------------------------+
SCTP ^ |
===========================|==|===========================
| v
Connectionless Packet Transfer Service (e.g., IP)
Notes:
1) When converting user messages into DATA chunks, an endpoint
will fragment user messages larger than the current association
path MTU into multiple DATA chunks. The data receiver will
normally reassemble the fragmented message from DATA chunks
before delivery to the user (see Section 6.9 for details).
2) Multiple DATA and control chunks may be bundled by the sender
into a single SCTP packet for transmission, as long as the
final size of the packet does not exceed the current path MTU.
The receiver will unbundle the packet back into the original
chunks. Control chunks MUST come before DATA chunks in the
packet.
Figure 6: Illustration of User Data Transfer
The fragmentation and bundling mechanisms, as detailed in Sections
6.9 and 6.10, are OPTIONAL to implement by the data sender, but they
MUST be implemented by the data receiver, i.e., an endpoint MUST
properly receive and process bundled or fragmented data.
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RFC 2960 Stream Control Transmission Protocol October 20006.1 Transmission of DATA Chunks
This document is specified as if there is a single retransmission
timer per destination transport address, but implementations MAY have
a retransmission timer for each DATA chunk.
The following general rules MUST be applied by the data sender for
transmission and/or retransmission of outbound DATA chunks:
A) At any given time, the data sender MUST NOT transmit new data to
any destination transport address if its peer's rwnd indicates
that the peer has no buffer space (i.e. rwnd is 0, see Section6.2.1). However, regardless of the value of rwnd (including if it
is 0), the data sender can always have one DATA chunk in flight to
the receiver if allowed by cwnd (see rule B below). This rule
allows the sender to probe for a change in rwnd that the sender
missed due to the SACK having been lost in transit from the data
receiver to the data sender.
B) At any given time, the sender MUST NOT transmit new data to a
given transport address if it has cwnd or more bytes of data
outstanding to that transport address.
C) When the time comes for the sender to transmit, before sending new
DATA chunks, the sender MUST first transmit any outstanding DATA
chunks which are marked for retransmission (limited by the current
cwnd).
D) Then, the sender can send out as many new DATA chunks as Rule A
and Rule B above allow.
Multiple DATA chunks committed for transmission MAY be bundled in a
single packet. Furthermore, DATA chunks being retransmitted MAY be
bundled with new DATA chunks, as long as the resulting packet size
does not exceed the path MTU. A ULP may request that no bundling is
performed but this should only turn off any delays that a SCTP
implementation may be using to increase bundling efficiency. It does
not in itself stop all bundling from occurring (i.e. in case of
congestion or retransmission).
Before an endpoint transmits a DATA chunk, if any received DATA
chunks have not been acknowledged (e.g., due to delayed ack), the
sender should create a SACK and bundle it with the outbound DATA
chunk, as long as the size of the final SCTP packet does not exceed
the current MTU. See Section 6.2.
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RFC 2960 Stream Control Transmission Protocol October 2000
IMPLEMENTATION NOTE: When the window is full (i.e., transmission is
disallowed by Rule A and/or Rule B), the sender MAY still accept send
requests from its upper layer, but MUST transmit no more DATA chunks
until some or all of the outstanding DATA chunks are acknowledged and
transmission is allowed by Rule A and Rule B again.
Whenever a transmission or retransmission is made to any address, if
the T3-rtx timer of that address is not currently running, the sender
MUST start that timer. If the timer for that address is already
running, the sender MUST restart the timer if the earliest (i.e.,
lowest TSN) outstanding DATA chunk sent to that address is being
retransmitted. Otherwise, the data sender MUST NOT restart the
timer.
When starting or restarting the T3-rtx timer, the timer value must be
adjusted according to the timer rules defined in Sections 6.3.2, and
6.3.3.
Note: The data sender SHOULD NOT use a TSN that is more than 2**31 -
1 above the beginning TSN of the current send window.
6.2 Acknowledgement on Reception of DATA Chunks
The SCTP endpoint MUST always acknowledge the reception of each valid
DATA chunk.
The guidelines on delayed acknowledgement algorithm specified in
Section 4.2 of [RFC2581] SHOULD be followed. Specifically, an
acknowledgement SHOULD be generated for at least every second packet
(not every second DATA chunk) received, and SHOULD be generated
within 200 ms of the arrival of any unacknowledged DATA chunk. In
some situations it may be beneficial for an SCTP transmitter to be
more conservative than the algorithms detailed in this document
allow. However, an SCTP transmitter MUST NOT be more aggressive than
the following algorithms allow.
A SCTP receiver MUST NOT generate more than one SACK for every
incoming packet, other than to update the offered window as the
receiving application consumes new data.
IMPLEMENTATION NOTE: The maximum delay for generating an
acknowledgement may be configured by the SCTP administrator, either
statically or dynamically, in order to meet the specific timing
requirement of the protocol being carried.
An implementation MUST NOT allow the maximum delay to be configured
to be more than 500 ms. In other words an implementation MAY lower
this value below 500ms but MUST NOT raise it above 500ms.
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Acknowledgements MUST be sent in SACK chunks unless shutdown was
requested by the ULP in which case an endpoint MAY send an
acknowledgement in the SHUTDOWN chunk. A SACK chunk can acknowledge
the reception of multiple DATA chunks. See Section 3.3.4 for SACK
chunk format. In particular, the SCTP endpoint MUST fill in the
Cumulative TSN Ack field to indicate the latest sequential TSN (of a
valid DATA chunk) it has received. Any received DATA chunks with TSN
greater than the value in the Cumulative TSN Ack field SHOULD also be
reported in the Gap Ack Block fields.
Note: The SHUTDOWN chunk does not contain Gap Ack Block fields.
Therefore, the endpoint should use a SACK instead of the SHUTDOWN
chunk to acknowledge DATA chunks received out of order .
When a packet arrives with duplicate DATA chunk(s) and with no new
DATA chunk(s), the endpoint MUST immediately send a SACK with no
delay. If a packet arrives with duplicate DATA chunk(s) bundled with
new DATA chunks, the endpoint MAY immediately send a SACK. Normally
receipt of duplicate DATA chunks will occur when the original SACK
chunk was lost and the peer's RTO has expired. The duplicate TSN
number(s) SHOULD be reported in the SACK as duplicate.
When an endpoint receives a SACK, it MAY use the Duplicate TSN
information to determine if SACK loss is occurring. Further use of
this data is for future study.
The data receiver is responsible for maintaining its receive buffers.
The data receiver SHOULD notify the data sender in a timely manner of
changes in its ability to receive data. How an implementation
manages its receive buffers is dependent on many factors (e.g.,
Operating System, memory management system, amount of memory, etc.).
However, the data sender strategy defined in Section 6.2.1 is based
on the assumption of receiver operation similar to the following:
A) At initialization of the association, the endpoint tells the
peer how much receive buffer space it has allocated to the
association in the INIT or INIT ACK. The endpoint sets a_rwnd
to this value.
B) As DATA chunks are received and buffered, decrement a_rwnd by
the number of bytes received and buffered. This is, in effect,
closing rwnd at the data sender and restricting the amount of
data it can transmit.
C) As DATA chunks are delivered to the ULP and released from the
receive buffers, increment a_rwnd by the number of bytes
delivered to the upper layer. This is, in effect, opening up
rwnd on the data sender and allowing it to send more data. The
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data receiver SHOULD NOT increment a_rwnd unless it has
released bytes from its receive buffer. For example, if the
receiver is holding fragmented DATA chunks in a reassembly
queue, it should not increment a_rwnd.
D) When sending a SACK, the data receiver SHOULD place the current
value of a_rwnd into the a_rwnd field. The data receiver
SHOULD take into account that the data sender will not
retransmit DATA chunks that are acked via the Cumulative TSN
Ack (i.e., will drop from its retransmit queue).
Under certain circumstances, the data receiver may need to drop DATA
chunks that it has received but hasn't released from its receive
buffers (i.e., delivered to the ULP). These DATA chunks may have
been acked in Gap Ack Blocks. For example, the data receiver may be
holding data in its receive buffers while reassembling a fragmented
user message from its peer when it runs out of receive buffer space.
It may drop these DATA chunks even though it has acknowledged them in
Gap Ack Blocks. If a data receiver drops DATA chunks, it MUST NOT
include them in Gap Ack Blocks in subsequent SACKs until they are
received again via retransmission. In addition, the endpoint should
take into account the dropped data when calculating its a_rwnd.
An endpoint SHOULD NOT revoke a SACK and discard data. Only in
extreme circumstance should an endpoint use this procedure (such as
out of buffer space). The data receiver should take into account
that dropping data that has been acked in Gap Ack Blocks can result
in suboptimal retransmission strategies in the data sender and thus
in suboptimal performance.
The following example illustrates the use of delayed
acknowledgements:
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Endpoint A Endpoint Z
{App sends 3 messages; strm 0}
DATA [TSN=7,Strm=0,Seq=3] ------------> (ack delayed)
(Start T3-rtx timer)
DATA [TSN=8,Strm=0,Seq=4] ------------> (send ack)
/------- SACK [TSN Ack=8,block=0]
(cancel T3-rtx timer) <-----/
DATA [TSN=9,Strm=0,Seq=5] ------------> (ack delayed)
(Start T3-rtx timer)
...
{App sends 1 message; strm 1}
(bundle SACK with DATA)
/----- SACK [TSN Ack=9,block=0] \
/ DATA [TSN=6,Strm=1,Seq=2]
(cancel T3-rtx timer) <------/ (Start T3-rtx timer)
(ack delayed)
(send ack)
SACK [TSN Ack=6,block=0] -------------> (cancel T3-rtx timer)
Figure 7: Delayed Acknowledgment Example
If an endpoint receives a DATA chunk with no user data (i.e., the
Length field is set to 16) it MUST send an ABORT with error cause set
to "No User Data".
An endpoint SHOULD NOT send a DATA chunk with no user data part.
6.2.1 Processing a Received SACK
Each SACK an endpoint receives contains an a_rwnd value. This value
represents the amount of buffer space the data receiver, at the time
of transmitting the SACK, has left of its total receive buffer space
(as specified in the INIT/INIT ACK). Using a_rwnd, Cumulative TSN
Ack and Gap Ack Blocks, the data sender can develop a representation
of the peer's receive buffer space.
One of the problems the data sender must take into account when
processing a SACK is that a SACK can be received out of order. That
is, a SACK sent by the data receiver can pass an earlier SACK and be
received first by the data sender. If a SACK is received out of
order, the data sender can develop an incorrect view of the peer's
receive buffer space.
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Since there is no explicit identifier that can be used to detect
out-of-order SACKs, the data sender must use heuristics to determine
if a SACK is new.
An endpoint SHOULD use the following rules to calculate the rwnd,
using the a_rwnd value, the Cumulative TSN Ack and Gap Ack Blocks in
a received SACK.
A) At the establishment of the association, the endpoint initializes
the rwnd to the Advertised Receiver Window Credit (a_rwnd) the
peer specified in the INIT or INIT ACK.
B) Any time a DATA chunk is transmitted (or retransmitted) to a peer,
the endpoint subtracts the data size of the chunk from the rwnd of
that peer.
C) Any time a DATA chunk is marked for retransmission (via either
T3-rtx timer expiration (Section 6.3.3)or via fast retransmit
(Section 7.2.4)), add the data size of those chunks to the rwnd.
Note: If the implementation is maintaining a timer on each DATA
chunk then only DATA chunks whose timer expired would be marked
for retransmission.
D) Any time a SACK arrives, the endpoint performs the following:
i) If Cumulative TSN Ack is less than the Cumulative TSN Ack
Point, then drop the SACK. Since Cumulative TSN Ack is
monotonically increasing, a SACK whose Cumulative TSN Ack is
less than the Cumulative TSN Ack Point indicates an out-of-
order SACK.
ii) Set rwnd equal to the newly received a_rwnd minus the
number of bytes still outstanding after processing the
Cumulative TSN Ack and the Gap Ack Blocks.
iii) If the SACK is missing a TSN that was previously
acknowledged via a Gap Ack Block (e.g., the data receiver
reneged on the data), then mark the corresponding DATA chunk as
available for retransmit: Mark it as missing for fast
retransmit as described in Section 7.2.4 and if no retransmit
timer is running for the destination address to which the DATA
chunk was originally transmitted, then T3-rtx is started for
that destination address.
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RFC 2960 Stream Control Transmission Protocol October 20006.3 Management of Retransmission Timer
An SCTP endpoint uses a retransmission timer T3-rtx to ensure data
delivery in the absence of any feedback from its peer. The duration
of this timer is referred to as RTO (retransmission timeout).
When an endpoint's peer is multi-homed, the endpoint will calculate a
separate RTO for each different destination transport address of its
peer endpoint.
The computation and management of RTO in SCTP follows closely how TCP
manages its retransmission timer. To compute the current RTO, an
endpoint maintains two state variables per destination transport
address: SRTT (smoothed round-trip time) and RTTVAR (round-trip time
variation).
6.3.1 RTO Calculation
The rules governing the computation of SRTT, RTTVAR, and RTO are as
follows:
C1) Until an RTT measurement has been made for a packet sent to the
given destination transport address, set RTO to the protocol
parameter 'RTO.Initial'.
C2) When the first RTT measurement R is made, set SRTT <- R, RTTVAR
<- R/2, and RTO <- SRTT + 4 * RTTVAR.
C3) When a new RTT measurement R' is made, set
RTTVAR <- (1 - RTO.Beta) * RTTVAR + RTO.Beta * |SRTT - R'| SRTT
<- (1 - RTO.Alpha) * SRTT + RTO.Alpha * R'
Note: The value of SRTT used in the update to RTTVAR is its value
before updating SRTT itself using the second assignment.
After the computation, update RTO <- SRTT + 4 * RTTVAR.
C4) When data is in flight and when allowed by rule C5 below, a new
RTT measurement MUST be made each round trip. Furthermore, new
RTT measurements SHOULD be made no more than once per round-trip
for a given destination transport address. There are two reasons
for this recommendation: First, it appears that measuring more
frequently often does not in practice yield any significant
benefit [ALLMAN99]; second, if measurements are made more often,
then the values of RTO.Alpha and RTO.Beta in rule C3 above should
be adjusted so that SRTT and RTTVAR still adjust to changes at
roughly the same rate (in terms of how many round trips it takes
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RFC 2960 Stream Control Transmission Protocol October 2000
them to reflect new values) as they would if making only one
measurement per round-trip and using RTO.Alpha and RTO.Beta as
given in rule C3. However, the exact nature of these adjustments
remains a research issue.
C5) Karn's algorithm: RTT measurements MUST NOT be made using packets
that were retransmitted (and thus for which it is ambiguous
whether the reply was for the first instance of the packet or a
later instance).
C6) Whenever RTO is computed, if it is less than RTO.Min seconds then
it is rounded up to RTO.Min seconds. The reason for this rule is
that RTOs that do not have a high minimum value are susceptible
to unnecessary timeouts [ALLMAN99].
C7) A maximum value may be placed on RTO provided it is at least
RTO.max seconds.
There is no requirement for the clock granularity G used for
computing RTT measurements and the different state variables, other
than:
G1) Whenever RTTVAR is computed, if RTTVAR = 0, then adjust RTTVAR <-
G.
Experience [ALLMAN99] has shown that finer clock granularities (<=
100 msec) perform somewhat better than more coarse granularities.
6.3.2 Retransmission Timer Rules
The rules for managing the retransmission timer are as follows:
R1) Every time a DATA chunk is sent to any address (including a
retransmission), if the T3-rtx timer of that address is not
running, start it running so that it will expire after the RTO of
that address. The RTO used here is that obtained after any
doubling due to previous T3-rtx timer expirations on the
corresponding destination address as discussed in rule E2 below.
R2) Whenever all outstanding data sent to an address have been
acknowledged, turn off the T3-rtx timer of that address.
R3) Whenever a SACK is received that acknowledges the DATA chunk with
the earliest outstanding TSN for that address, restart T3-rtx
timer for that address with its current RTO (if there is still
outstanding data on that address).
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RFC 2960 Stream Control Transmission Protocol October 2000
R4) Whenever a SACK is received missing a TSN that was previously
acknowledged via a Gap Ack Block, start T3-rtx for the
destination address to which the DATA chunk was originally
transmitted if it is not already running.
The following example shows the use of various timer rules (assuming
the receiver uses delayed acks).
Endpoint A Endpoint Z
{App begins to send}
Data [TSN=7,Strm=0,Seq=3] ------------> (ack delayed)
(Start T3-rtx timer)
{App sends 1 message; strm 1}
(bundle ack with data)
DATA [TSN=8,Strm=0,Seq=4] ----\ /-- SACK [TSN Ack=7,Block=0]
\ / DATA [TSN=6,Strm=1,Seq=2]
\ / (Start T3-rtx timer)
\
/ \
(Re-start T3-rtx timer) <------/ \--> (ack delayed)
(ack delayed)
{send ack}
SACK [TSN Ack=6,Block=0] --------------> (Cancel T3-rtx timer)
..
(send ack)
(Cancel T3-rtx timer) <-------------- SACK [TSN Ack=8,Block=0]
Figure 8 - Timer Rule Examples
6.3.3 Handle T3-rtx Expiration
Whenever the retransmission timer T3-rtx expires for a destination
address, do the following:
E1) For the destination address for which the timer expires, adjust
its ssthresh with rules defined in Section 7.2.3 and set the cwnd
<- MTU.
E2) For the destination address for which the timer expires, set RTO
<- RTO * 2 ("back off the timer"). The maximum value discussed
in rule C7 above (RTO.max) may be used to provide an upper bound
to this doubling operation.
E3) Determine how many of the earliest (i.e., lowest TSN) outstanding
DATA chunks for the address for which the T3-rtx has expired will
fit into a single packet, subject to the MTU constraint for the
path corresponding to the destination transport address to which
the retransmission is being sent (this may be different from the
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RFC 2960 Stream Control Transmission Protocol October 2000
address for which the timer expires [see Section 6.4]). Call
this value K. Bundle and retransmit those K DATA chunks in a
single packet to the destination endpoint.
E4) Start the retransmission timer T3-rtx on the destination address
to which the retransmission is sent, if rule R1 above indicates
to do so. The RTO to be used for starting T3-rtx should be the
one for the destination address to which the retransmission is
sent, which, when the receiver is multi-homed, may be different
from the destination address for which the timer expired (see
Section 6.4 below).
After retransmitting, once a new RTT measurement is obtained (which
can happen only when new data has been sent and acknowledged, per
rule C5, or for a measurement made from a HEARTBEAT [see Section8.3]), the computation in rule C3 is performed, including the
computation of RTO, which may result in "collapsing" RTO back down
after it has been subject to doubling (rule E2).
Note: Any DATA chunks that were sent to the address for which the
T3-rtx timer expired but did not fit in one MTU (rule E3 above),
should be marked for retransmission and sent as soon as cwnd allows
(normally when a SACK arrives).
The final rule for managing the retransmission timer concerns
failover (see Section 6.4.1):
F1) Whenever an endpoint switches from the current destination
transport address to a different one, the current retransmission
timers are left running. As soon as the endpoint transmits a
packet containing DATA chunk(s) to the new transport address,
start the timer on that transport address, using the RTO value of
the destination address to which the data is being sent, if rule
R1 indicates to do so.
6.4 Multi-homed SCTP Endpoints
An SCTP endpoint is considered multi-homed if there are more than one
transport address that can be used as a destination address to reach
that endpoint.
Moreover, the ULP of an endpoint shall select one of the multiple
destination addresses of a multi-homed peer endpoint as the primary
path (see Sections 5.1.2 and 10.1 for details).
By default, an endpoint SHOULD always transmit to the primary path,
unless the SCTP user explicitly specifies the destination transport
address (and possibly source transport address) to use.
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An endpoint SHOULD transmit reply chunks (e.g., SACK, HEARTBEAT ACK,
etc.) to the same destination transport address from which it
received the DATA or control chunk to which it is replying. This
rule should also be followed if the endpoint is bundling DATA chunks
together with the reply chunk.
However, when acknowledging multiple DATA chunks received in packets
from different source addresses in a single SACK, the SACK chunk may
be transmitted to one of the destination transport addresses from
which the DATA or control chunks being acknowledged were received.
When a receiver of a duplicate DATA chunk sends a SACK to a multi-
homed endpoint it MAY be beneficial to vary the destination address
and not use the source address of the DATA chunk. The reason being
that receiving a duplicate from a multi-homed endpoint might indicate
that the return path (as specified in the source address of the DATA
chunk) for the SACK is broken.
Furthermore, when its peer is multi-homed, an endpoint SHOULD try to
retransmit a chunk to an active destination transport address that is
different from the last destination address to which the DATA chunk
was sent.
Retransmissions do not affect the total outstanding data count.
However, if the DATA chunk is retransmitted onto a different
destination address, both the outstanding data counts on the new
destination address and the old destination address to which the data
chunk was last sent shall be adjusted accordingly.
6.4.1 Failover from Inactive Destination Address
Some of the transport addresses of a multi-homed SCTP endpoint may
become inactive due to either the occurrence of certain error
conditions (see Section 8.2) or adjustments from SCTP user.
When there is outbound data to send and the primary path becomes
inactive (e.g., due to failures), or where the SCTP user explicitly
requests to send data to an inactive destination transport address,
before reporting an error to its ULP, the SCTP endpoint should try to
send the data to an alternate active destination transport address if
one exists.
When retransmitting data, if the endpoint is multi-homed, it should
consider each source-destination address pair in its retransmission
selection policy. When retransmitting the endpoint should attempt to
pick the most divergent source-destination pair from the original
source-destination pair to which the packet was transmitted.
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RFC 2960 Stream Control Transmission Protocol October 2000
Note: Rules for picking the most divergent source-destination pair
are an implementation decision and is not specified within this
document.
6.5 Stream Identifier and Stream Sequence Number
Every DATA chunk MUST carry a valid stream identifier. If an
endpoint receives a DATA chunk with an invalid stream identifier, it
shall acknowledge the reception of the DATA chunk following the
normal procedure, immediately send an ERROR chunk with cause set to
"Invalid Stream Identifier" (see Section 3.3.10) and discard the DATA
chunk. The endpoint may bundle the ERROR chunk in the same packet as
the SACK as long as the ERROR follows the SACK.
The stream sequence number in all the streams shall start from 0 when
the association is established. Also, when the stream sequence
number reaches the value 65535 the next stream sequence number shall
be set to 0.
6.6 Ordered and Unordered Delivery
Within a stream, an endpoint MUST deliver DATA chunks received with
the U flag set to 0 to the upper layer according to the order of
their stream sequence number. If DATA chunks arrive out of order of
their stream sequence number, the endpoint MUST hold the received
DATA chunks from delivery to the ULP until they are re-ordered.
However, an SCTP endpoint can indicate that no ordered delivery is
required for a particular DATA chunk transmitted within the stream by
setting the U flag of the DATA chunk to 1.
When an endpoint receives a DATA chunk with the U flag set to 1, it
must bypass the ordering mechanism and immediately deliver the data
to the upper layer (after re-assembly if the user data is fragmented
by the data sender).
This provides an effective way of transmitting "out-of-band" data in
a given stream. Also, a stream can be used as an "unordered" stream
by simply setting the U flag to 1 in all DATA chunks sent through
that stream.
IMPLEMENTATION NOTE: When sending an unordered DATA chunk, an
implementation may choose to place the DATA chunk in an outbound
packet that is at the head of the outbound transmission queue if
possible.
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RFC 2960 Stream Control Transmission Protocol October 2000
The 'Stream Sequence Number' field in a DATA chunk with U flag set to
1 has no significance. The sender can fill it with arbitrary value,
but the receiver MUST ignore the field.
Note: When transmitting ordered and unordered data, an endpoint does
not increment its Stream Sequence Number when transmitting a DATA
chunk with U flag set to 1.
6.7 Report Gaps in Received DATA TSNs
Upon the reception of a new DATA chunk, an endpoint shall examine the
continuity of the TSNs received. If the endpoint detects a gap in
the received DATA chunk sequence, it SHOULD send a SACK with Gap Ack
Blocks immediately. The data receiver continues sending a SACK after
receipt of each SCTP packet that doesn't fill the gap.
Based on the Gap Ack Block from the received SACK, the endpoint can
calculate the missing DATA chunks and make decisions on whether to
retransmit them (see Section 6.2.1 for details).
Multiple gaps can be reported in one single SACK (see Section 3.3.4).
When its peer is multi-homed, the SCTP endpoint SHOULD always try to
send the SACK to the same destination address from which the last
DATA chunk was received.
Upon the reception of a SACK, the endpoint MUST remove all DATA
chunks which have been acknowledged by the SACK's Cumulative TSN Ack
from its transmit queue. The endpoint MUST also treat all the DATA
chunks with TSNs not included in the Gap Ack Blocks reported by the
SACK as "missing". The number of "missing" reports for each
outstanding DATA chunk MUST be recorded by the data sender in order
to make retransmission decisions. See Section 7.2.4 for details.
The following example shows the use of SACK to report a gap.
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RFC 2960 Stream Control Transmission Protocol October 2000
Endpoint A Endpoint Z
{App sends 3 messages; strm 0}
DATA [TSN=6,Strm=0,Seq=2] ---------------> (ack delayed)
(Start T3-rtx timer)
DATA [TSN=7,Strm=0,Seq=3] --------> X (lost)
DATA [TSN=8,Strm=0,Seq=4] ---------------> (gap detected,
immediately send ack)
/----- SACK [TSN Ack=6,Block=1,
/ Strt=2,End=2]
<-----/
(remove 6 from out-queue,
and mark 7 as "1" missing report)
Figure 9 - Reporting a Gap using SACK
The maximum number of Gap Ack Blocks that can be reported within a
single SACK chunk is limited by the current path MTU. When a single
SACK can not cover all the Gap Ack Blocks needed to be reported due
to the MTU limitation, the endpoint MUST send only one SACK,
reporting the Gap Ack Blocks from the lowest to highest TSNs, within
the size limit set by the MTU, and leave the remaining highest TSN
numbers unacknowledged.
6.8 Adler-32 Checksum Calculation
When sending an SCTP packet, the endpoint MUST strengthen the data
integrity of the transmission by including the Adler-32 checksum
value calculated on the packet, as described below.
After the packet is constructed (containing the SCTP common header
and one or more control or DATA chunks), the transmitter shall:
1) Fill in the proper Verification Tag in the SCTP common header and
initialize the checksum field to 0's.
2) Calculate the Adler-32 checksum of the whole packet, including the
SCTP common header and all the chunks. Refer to appendix B for
details of the Adler-32 algorithm. And,
3) Put the resultant value into the checksum field in the common
header, and leave the rest of the bits unchanged.
When an SCTP packet is received, the receiver MUST first check the
Adler-32 checksum:
1) Store the received Adler-32 checksum value aside,
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RFC 2960 Stream Control Transmission Protocol October 2000
2) Replace the 32 bits of the checksum field in the received SCTP
packet with all '0's and calculate an Adler-32 checksum value of
the whole received packet. And,
3) Verify that the calculated Adler-32 checksum is the same as the
received Adler-32 checksum. If not, the receiver MUST treat the
packet as an invalid SCTP packet.
The default procedure for handling invalid SCTP packets is to
silently discard them.
6.9 Fragmentation and Reassembly
An endpoint MAY support fragmentation when sending DATA chunks, but
MUST support reassembly when receiving DATA chunks. If an endpoint
supports fragmentation, it MUST fragment a user message if the size
of the user message to be sent causes the outbound SCTP packet size
to exceed the current MTU. If an implementation does not support
fragmentation of outbound user messages, the endpoint must return an
error to its upper layer and not attempt to send the user message.
IMPLEMENTATION NOTE: In this error case, the Send primitive
discussed in Section 10.1 would need to return an error to the upper
layer.
If its peer is multi-homed, the endpoint shall choose a size no
larger than the association Path MTU. The association Path MTU is
the smallest Path MTU of all destination addresses.
Note: Once a message is fragmented it cannot be re-fragmented.
Instead if the PMTU has been reduced, then IP fragmentation must be
used. Please see Section 7.3 for details of PMTU discovery.
When determining when to fragment, the SCTP implementation MUST take
into account the SCTP packet header as well as the DATA chunk
header(s). The implementation MUST also take into account the space
required for a SACK chunk if bundling a SACK chunk with the DATA
chunk.
Fragmentation takes the following steps:
1) The data sender MUST break the user message into a series of DATA
chunks such that each chunk plus SCTP overhead fits into an IP
datagram smaller than or equal to the association Path MTU.
2) The transmitter MUST then assign, in sequence, a separate TSN to
each of the DATA chunks in the series. The transmitter assigns
the same SSN to each of the DATA chunks. If the user indicates
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RFC 2960 Stream Control Transmission Protocol October 2000
that the user message is to be delivered using unordered delivery,
then the U flag of each DATA chunk of the user message MUST be set
to 1.
3) The transmitter MUST also set the B/E bits of the first DATA chunk
in the series to '10', the B/E bits of the last DATA chunk in the
series to '01', and the B/E bits of all other DATA chunks in the
series to '00'.
An endpoint MUST recognize fragmented DATA chunks by examining the
B/E bits in each of the received DATA chunks, and queue the
fragmented DATA chunks for re-assembly. Once the user message is
reassembled, SCTP shall pass the re-assembled user message to the
specific stream for possible re-ordering and final dispatching.
Note: If the data receiver runs out of buffer space while still
waiting for more fragments to complete the re-assembly of the
message, it should dispatch part of its inbound message through a
partial delivery API (see Section 10), freeing some of its receive
buffer space so that the rest of the message may be received.
6.10 Bundling
An endpoint bundles chunks by simply including multiple chunks in one
outbound SCTP packet. The total size of the resultant IP datagram,
including the SCTP packet and IP headers, MUST be less or equal to
the current Path MTU.
If its peer endpoint is multi-homed, the sending endpoint shall
choose a size no larger than the latest MTU of the current primary
path.
When bundling control chunks with DATA chunks, an endpoint MUST place
control chunks first in the outbound SCTP packet. The transmitter
MUST transmit DATA chunks within a SCTP packet in increasing order of
TSN.
Note: Since control chunks must be placed first in a packet and
since DATA chunks must be transmitted before SHUTDOWN or SHUTDOWN ACK
chunks, DATA chunks cannot be bundled with SHUTDOWN or SHUTDOWN ACK
chunks.
Partial chunks MUST NOT be placed in an SCTP packet.
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RFC 2960 Stream Control Transmission Protocol October 2000
An endpoint MUST process received chunks in their order in the
packet. The receiver uses the chunk length field to determine the end
of a chunk and beginning of the next chunk taking account of the fact
that all chunks end on a 4 byte boundary. If the receiver detects a
partial chunk, it MUST drop the chunk.
An endpoint MUST NOT bundle INIT, INIT ACK or SHUTDOWN COMPLETE with
any other chunks.
7. Congestion control
Congestion control is one of the basic functions in SCTP. For some
applications, it may be likely that adequate resources will be
allocated to SCTP traffic to assure prompt delivery of time-critical
data - thus it would appear to be unlikely, during normal operations,
that transmissions encounter severe congestion conditions. However
SCTP must operate under adverse operational conditions, which can
develop upon partial network failures or unexpected traffic surges.
In such situations SCTP must follow correct congestion control steps
to recover from congestion quickly in order to get data delivered as
soon as possible. In the absence of network congestion, these
preventive congestion control algorithms should show no impact on the
protocol performance.
IMPLEMENTATION NOTE: As far as its specific performance requirements
are met, an implementation is always allowed to adopt a more
conservative congestion control algorithm than the one defined below.
The congestion control algorithms used by SCTP are based on
[RFC2581]. This section describes how the algorithms defined in
RFC2581 are adapted for use in SCTP. We first list differences in
protocol designs between TCP and SCTP, and then describe SCTP's
congestion control scheme. The description will use the same
terminology as in TCP congestion control whenever appropriate.
SCTP congestion control is always applied to the entire association,
and not to individual streams.
7.1 SCTP Differences from TCP Congestion control
Gap Ack Blocks in the SCTP SACK carry the same semantic meaning as
the TCP SACK. TCP considers the information carried in the SACK as
advisory information only. SCTP considers the information carried in
the Gap Ack Blocks in the SACK chunk as advisory. In SCTP, any DATA
chunk that has been acknowledged by SACK, including DATA that arrived
at the receiving end out of order, are not considered fully delivered
until the Cumulative TSN Ack Point passes the TSN of the DATA chunk
(i.e., the DATA chunk has been acknowledged by the Cumulative TSN Ack
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RFC 2960 Stream Control Transmission Protocol October 2000
field in the SACK). Consequently, the value of cwnd controls the
amount of outstanding data, rather than (as in the case of non-SACK
TCP) the upper bound between the highest acknowledged sequence number
and the latest DATA chunk that can be sent within the congestion
window. SCTP SACK leads to different implementations of fast-
retransmit and fast-recovery than non-SACK TCP. As an example see
[FALL96].
The biggest difference between SCTP and TCP, however, is multi-
homing. SCTP is designed to establish robust communication
associations between two endpoints each of which may be reachable by
more than one transport address. Potentially different addresses may
lead to different data paths between the two endpoints, thus ideally
one may need a separate set of congestion control parameters for each
of the paths. The treatment here of congestion control for multi-
homed receivers is new with SCTP and may require refinement in the
future. The current algorithms make the following assumptions:
o The sender usually uses the same destination address until being
instructed by the upper layer otherwise; however, SCTP may change
to an alternate destination in the event an address is marked
inactive (see Section 8.2). Also, SCTP may retransmit to a
different transport address than the original transmission.
o The sender keeps a separate congestion control parameter set for
each of the destination addresses it can send to (not each
source-destination pair but for each destination). The parameters
should decay if the address is not used for a long enough time
period.
o For each of the destination addresses, an endpoint does slow-start
upon the first transmission to that address.
Note: TCP guarantees in-sequence delivery of data to its upper-layer
protocol within a single TCP session. This means that when TCP
notices a gap in the received sequence number, it waits until the gap
is filled before delivering the data that was received with sequence
numbers higher than that of the missing data. On the other hand,
SCTP can deliver data to its upper-layer protocol even if there is a
gap in TSN if the Stream Sequence Numbers are in sequence for a
particular stream (i.e., the missing DATA chunks are for a different
stream) or if unordered delivery is indicated. Although this does
not affect cwnd, it might affect rwnd calculation.
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RFC 2960 Stream Control Transmission Protocol October 20007.2 SCTP Slow-Start and Congestion Avoidance
The slow start and congestion avoidance algorithms MUST be used by an
endpoint to control the amount of data being injected into the
network. The congestion control in SCTP is employed in regard to the
association, not to an individual stream. In some situations it may
be beneficial for an SCTP sender to be more conservative than the
algorithms allow; however, an SCTP sender MUST NOT be more aggressive
than the following algorithms allow.
Like TCP, an SCTP endpoint uses the following three control variables
to regulate its transmission rate.
o Receiver advertised window size (rwnd, in bytes), which is set by
the receiver based on its available buffer space for incoming
packets.
Note: This variable is kept on the entire association.
o Congestion control window (cwnd, in bytes), which is adjusted by
the sender based on observed network conditions.
Note: This variable is maintained on a per-destination address
basis.
o Slow-start threshold (ssthresh, in bytes), which is used by the
sender to distinguish slow start and congestion avoidance phases.
Note: This variable is maintained on a per-destination address
basis.
SCTP also requires one additional control variable,
partial_bytes_acked, which is used during congestion avoidance phase
to facilitate cwnd adjustment.
Unlike TCP, an SCTP sender MUST keep a set of these control variables
cwnd, ssthresh and partial_bytes_acked for EACH destination address
of its peer (when its peer is multi-homed). Only one rwnd is kept
for the whole association (no matter if the peer is multi-homed or
has a single address).
7.2.1 Slow-Start
Beginning data transmission into a network with unknown conditions or
after a sufficiently long idle period requires SCTP to probe the
network to determine the available capacity. The slow start
algorithm is used for this purpose at the beginning of a transfer, or
after repairing loss detected by the retransmission timer.
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RFC 2960 Stream Control Transmission Protocol October 2000
o The initial cwnd before DATA transmission or after a sufficiently
long idle period MUST be <= 2*MTU.
o The initial cwnd after a retransmission timeout MUST be no more
than 1*MTU.
o The initial value of ssthresh MAY be arbitrarily high (for
example, implementations MAY use the size of the receiver
advertised window).
o Whenever cwnd is greater than zero, the endpoint is allowed to
have cwnd bytes of data outstanding on that transport address.
o When cwnd is less than or equal to ssthresh an SCTP endpoint MUST
use the slow start algorithm to increase cwnd (assuming the
current congestion window is being fully utilized). If an
incoming SACK advances the Cumulative TSN Ack Point, cwnd MUST be
increased by at most the lesser of 1) the total size of the
previously outstanding DATA chunk(s) acknowledged, and 2) the
destination's path MTU. This protects against the ACK-Splitting
attack outlined in [SAVAGE99].
In instances where its peer endpoint is multi-homed, if an endpoint
receives a SACK that advances its Cumulative TSN Ack Point, then it
should update its cwnd (or cwnds) apportioned to the destination
addresses to which it transmitted the acknowledged data. However if
the received SACK does not advance the Cumulative TSN Ack Point, the
endpoint MUST NOT adjust the cwnd of any of the destination
addresses.
Because an endpoint's cwnd is not tied to its Cumulative TSN Ack
Point, as duplicate SACKs come in, even though they may not advance
the Cumulative TSN Ack Point an endpoint can still use them to clock
out new data. That is, the data newly acknowledged by the SACK
diminishes the amount of data now in flight to less than cwnd; and so
the current, unchanged value of cwnd now allows new data to be sent.
On the other hand, the increase of cwnd must be tied to the
Cumulative TSN Ack Point advancement as specified above. Otherwise
the duplicate SACKs will not only clock out new data, but also will
adversely clock out more new data than what has just left the
network, during a time of possible congestion.
o When the endpoint does not transmit data on a given transport
address, the cwnd of the transport address should be adjusted to
max(cwnd/2, 2*MTU) per RTO.
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RFC 2960 Stream Control Transmission Protocol October 20007.2.2 Congestion Avoidance
When cwnd is greater than ssthresh, cwnd should be incremented by
1*MTU per RTT if the sender has cwnd or more bytes of data
outstanding for the corresponding transport address.
In practice an implementation can achieve this goal in the following
way:
o partial_bytes_acked is initialized to 0.
o Whenever cwnd is greater than ssthresh, upon each SACK arrival
that advances the Cumulative TSN Ack Point, increase
partial_bytes_acked by the total number of bytes of all new chunks
acknowledged in that SACK including chunks acknowledged by the new
Cumulative TSN Ack and by Gap Ack Blocks.
o When partial_bytes_acked is equal to or greater than cwnd and
before the arrival of the SACK the sender had cwnd or more bytes
of data outstanding (i.e., before arrival of the SACK, flightsize
was greater than or equal to cwnd), increase cwnd by MTU, and
reset partial_bytes_acked to (partial_bytes_acked - cwnd).
o Same as in the slow start, when the sender does not transmit DATA
on a given transport address, the cwnd of the transport address
should be adjusted to max(cwnd / 2, 2*MTU) per RTO.
o When all of the data transmitted by the sender has been
acknowledged by the receiver, partial_bytes_acked is initialized
to 0.
7.2.3 Congestion Control
Upon detection of packet losses from SACK (see Section 7.2.4), An
endpoint should do the following:
ssthresh = max(cwnd/2, 2*MTU)
cwnd = ssthresh
Basically, a packet loss causes cwnd to be cut in half.
When the T3-rtx timer expires on an address, SCTP should perform slow
start by:
ssthresh = max(cwnd/2, 2*MTU)
cwnd = 1*MTU
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RFC 2960 Stream Control Transmission Protocol October 2000
and assure that no more than one SCTP packet will be in flight for
that address until the endpoint receives acknowledgement for
successful delivery of data to that address.
7.2.4 Fast Retransmit on Gap Reports
In the absence of data loss, an endpoint performs delayed
acknowledgement. However, whenever an endpoint notices a hole in the
arriving TSN sequence, it SHOULD start sending a SACK back every time
a packet arrives carrying data until the hole is filled.
Whenever an endpoint receives a SACK that indicates some TSN(s)
missing, it SHOULD wait for 3 further miss indications (via
subsequent SACK's) on the same TSN(s) before taking action with
regard to Fast Retransmit.
When the TSN(s) is reported as missing in the fourth consecutive
SACK, the data sender shall:
1) Mark the missing DATA chunk(s) for retransmission,
2) Adjust the ssthresh and cwnd of the destination address(es) to
which the missing DATA chunks were last sent, according to the
formula described in Section 7.2.3.
3) Determine how many of the earliest (i.e., lowest TSN) DATA chunks
marked for retransmission will fit into a single packet, subject
to constraint of the path MTU of the destination transport address
to which the packet is being sent. Call this value K. Retransmit
those K DATA chunks in a single packet.
4) Restart T3-rtx timer only if the last SACK acknowledged the lowest
outstanding TSN number sent to that address, or the endpoint is
retransmitting the first outstanding DATA chunk sent to that
address.
Note: Before the above adjustments, if the received SACK also
acknowledges new DATA chunks and advances the Cumulative TSN Ack
Point, the cwnd adjustment rules defined in Sections 7.2.1 and 7.2.2
must be applied first.
A straightforward implementation of the above keeps a counter for
each TSN hole reported by a SACK. The counter increments for each
consecutive SACK reporting the TSN hole. After reaching 4 and
starting the fast retransmit procedure, the counter resets to 0.
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Because cwnd in SCTP indirectly bounds the number of outstanding
TSN's, the effect of TCP fast-recovery is achieved automatically with
no adjustment to the congestion control window size.
7.3 Path MTU Discovery
[RFC1191] specifies "Path MTU Discovery", whereby an endpoint
maintains an estimate of the maximum transmission unit (MTU) along a
given Internet path and refrains from sending packets along that path
which exceed the MTU, other than occasional attempts to probe for a
change in the Path MTU (PMTU). RFC 1191 is thorough in its
discussion of the MTU discovery mechanism and strategies for
determining the current end-to-end MTU setting as well as detecting
changes in this value. [RFC1981] specifies the same mechanisms for
IPv6. An SCTP sender using IPv6 MUST use Path MTU Discovery unless
all packets are less than the minimum IPv6 MTU [RFC2460].
An endpoint SHOULD apply these techniques, and SHOULD do so on a
per-destination-address basis.
There are 4 ways in which SCTP differs from the description in RFC1191 of applying MTU discovery to TCP:
1) SCTP associations can span multiple addresses. An endpoint MUST
maintain separate MTU estimates for each destination address of
its peer.
2) Elsewhere in this document, when the term "MTU" is discussed, it
refers to the MTU associated with the destination address
corresponding to the context of the discussion.
3) Unlike TCP, SCTP does not have a notion of "Maximum Segment Size".
Accordingly, the MTU for each destination address SHOULD be
initialized to a value no larger than the link MTU for the local
interface to which packets for that remote destination address
will be routed.
4) Since data transmission in SCTP is naturally structured in terms
of TSNs rather than bytes (as is the case for TCP), the discussion
in Section 6.5 of RFC 1191 applies: When retransmitting an IP
datagram to a remote address for which the IP datagram appears too
large for the path MTU to that address, the IP datagram SHOULD be
retransmitted without the DF bit set, allowing it to possibly be
fragmented. Transmissions of new IP datagrams MUST have DF set.
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5) The sender should track an association PMTU which will be the
smallest PMTU discovered for all of the peer's destination
addresses. When fragmenting messages into multiple parts this
association PMTU should be used to calculate the size of each
fragment. This will allow retransmissions to be seamlessly sent
to an alternate address without encountering IP fragmentation.
Other than these differences, the discussion of TCP's use of MTU
discovery in RFCs 1191 and 1981 applies to SCTP on a per-
destination-address basis.
Note: For IPv6 destination addresses the DF bit does not exist,
instead the IP datagram must be fragmented as described in [RFC2460].
8. Fault Management8.1 Endpoint Failure Detection
An endpoint shall keep a counter on the total number of consecutive
retransmissions to its peer (including retransmissions to all the
destination transport addresses of the peer if it is multi-homed).
If the value of this counter exceeds the limit indicated in the
protocol parameter 'Association.Max.Retrans', the endpoint shall
consider the peer endpoint unreachable and shall stop transmitting
any more data to it (and thus the association enters the CLOSED
state). In addition, the endpoint shall report the failure to the
upper layer, and optionally report back all outstanding user data
remaining in its outbound queue. The association is automatically
closed when the peer endpoint becomes unreachable.
The counter shall be reset each time a DATA chunk sent to that peer
endpoint is acknowledged (by the reception of a SACK), or a
HEARTBEAT-ACK is received from the peer endpoint.
8.2 Path Failure Detection
When its peer endpoint is multi-homed, an endpoint should keep a
error counter for each of the destination transport addresses of the
peer endpoint.
Each time the T3-rtx timer expires on any address, or when a
HEARTBEAT sent to an idle address is not acknowledged within a RTO,
the error counter of that destination address will be incremented.
When the value in the error counter exceeds the protocol parameter
'Path.Max.Retrans' of that destination address, the endpoint should
mark the destination transport address as inactive, and a
notification SHOULD be sent to the upper layer.
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RFC 2960 Stream Control Transmission Protocol October 2000
When an outstanding TSN is acknowledged or a HEARTBEAT sent to that
address is acknowledged with a HEARTBEAT ACK, the endpoint shall
clear the error counter of the destination transport address to which
the DATA chunk was last sent (or HEARTBEAT was sent). When the peer
endpoint is multi-homed and the last chunk sent to it was a
retransmission to an alternate address, there exists an ambiguity as
to whether or not the acknowledgement should be credited to the
address of the last chunk sent. However, this ambiguity does not
seem to bear any significant consequence to SCTP behavior. If this
ambiguity is undesirable, the transmitter may choose not to clear the
error counter if the last chunk sent was a retransmission.
Note: When configuring the SCTP endpoint, the user should avoid
having the value of 'Association.Max.Retrans' larger than the
summation of the 'Path.Max.Retrans' of all the destination addresses
for the remote endpoint. Otherwise, all the destination addresses
may become inactive while the endpoint still considers the peer
endpoint reachable. When this condition occurs, how the SCTP chooses
to function is implementation specific.
When the primary path is marked inactive (due to excessive
retransmissions, for instance), the sender MAY automatically transmit
new packets to an alternate destination address if one exists and is
active. If more than one alternate address is active when the
primary path is marked inactive only ONE transport address SHOULD be
chosen and used as the new destination transport address.
8.3 Path Heartbeat
By default, an SCTP endpoint shall monitor the reachability of the
idle destination transport address(es) of its peer by sending a
HEARTBEAT chunk periodically to the destination transport
address(es).
A destination transport address is considered "idle" if no new chunk
which can be used for updating path RTT (usually including first
transmission DATA, INIT, COOKIE ECHO, HEARTBEAT etc.) and no
HEARTBEAT has been sent to it within the current heartbeat period of
that address. This applies to both active and inactive destination
addresses.
The upper layer can optionally initiate the following functions:
A) Disable heartbeat on a specific destination transport address of a
given association,
B) Change the HB.interval,
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RFC 2960 Stream Control Transmission Protocol October 2000
C) Re-enable heartbeat on a specific destination transport address of
a given association, and,
D) Request an on-demand HEARTBEAT on a specific destination transport
address of a given association.
The endpoint should increment the respective error counter of the
destination transport address each time a HEARTBEAT is sent to that
address and not acknowledged within one RTO.
When the value of this counter reaches the protocol parameter '
Path.Max.Retrans', the endpoint should mark the corresponding
destination address as inactive if it is not so marked, and may also
optionally report to the upper layer the change of reachability of
this destination address. After this, the endpoint should continue
HEARTBEAT on this destination address but should stop increasing the
counter.
The sender of the HEARTBEAT chunk should include in the Heartbeat
Information field of the chunk the current time when the packet is
sent out and the destination address to which the packet is sent.
IMPLEMENTATION NOTE: An alternative implementation of the heartbeat
mechanism that can be used is to increment the error counter variable
every time a HEARTBEAT is sent to a destination. Whenever a
HEARTBEAT ACK arrives, the sender SHOULD clear the error counter of
the destination that the HEARTBEAT was sent to. This in effect would
clear the previously stroked error (and any other error counts as
well).
The receiver of the HEARTBEAT should immediately respond with a
HEARTBEAT ACK that contains the Heartbeat Information field copied
from the received HEARTBEAT chunk.
Upon the receipt of the HEARTBEAT ACK, the sender of the HEARTBEAT
should clear the error counter of the destination transport address
to which the HEARTBEAT was sent, and mark the destination transport
address as active if it is not so marked. The endpoint may
optionally report to the upper layer when an inactive destination
address is marked as active due to the reception of the latest
HEARTBEAT ACK. The receiver of the HEARTBEAT ACK must also clear the
association overall error count as well (as defined in section 8.1).
The receiver of the HEARTBEAT ACK should also perform an RTT
measurement for that destination transport address using the time
value carried in the HEARTBEAT ACK chunk.
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RFC 2960 Stream Control Transmission Protocol October 2000
On an idle destination address that is allowed to heartbeat, a
HEARTBEAT chunk is RECOMMENDED to be sent once per RTO of that
destination address plus the protocol parameter 'HB.interval' , with
jittering of +/- 50%, and exponential back-off of the RTO if the
previous HEARTBEAT is unanswered.
A primitive is provided for the SCTP user to change the HB.interval
and turn on or off the heartbeat on a given destination address. The
heartbeat interval set by the SCTP user is added to the RTO of that
destination (including any exponential backoff). Only one heartbeat
should be sent each time the heartbeat timer expires (if multiple
destinations are idle). It is a implementation decision on how to
choose which of the candidate idle destinations to heartbeat to (if
more than one destination is idle).
Note: When tuning the heartbeat interval, there is a side effect that
SHOULD be taken into account. When this value is increased, i.e.
the HEARTBEAT takes longer, the detection of lost ABORT messages
takes longer as well. If a peer endpoint ABORTs the association for
any reason and the ABORT chunk is lost, the local endpoint will only
discover the lost ABORT by sending a DATA chunk or HEARTBEAT chunk
(thus causing the peer to send another ABORT). This must be
considered when tuning the HEARTBEAT timer. If the HEARTBEAT is
disabled only sending DATA to the association will discover a lost
ABORT from the peer.
8.4 Handle "Out of the blue" Packets
An SCTP packet is called an "out of the blue" (OOTB) packet if it is
correctly formed, i.e., passed the receiver's Adler-32 check (see
Section 6.8), but the receiver is not able to identify the
association to which this packet belongs.
The receiver of an OOTB packet MUST do the following:
1) If the OOTB packet is to or from a non-unicast address, silently
discard the packet. Otherwise,
2) If the OOTB packet contains an ABORT chunk, the receiver MUST
silently discard the OOTB packet and take no further action.
Otherwise,
3) If the packet contains an INIT chunk with a Verification Tag set
to '0', process it as described in Section 5.1. Otherwise,
4) If the packet contains a COOKIE ECHO in the first chunk, process
it as described in Section 5.1. Otherwise,
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5) If the packet contains a SHUTDOWN ACK chunk, the receiver should
respond to the sender of the OOTB packet with a SHUTDOWN COMPLETE.
When sending the SHUTDOWN COMPLETE, the receiver of the OOTB
packet must fill in the Verification Tag field of the outbound
packet with the Verification Tag received in the SHUTDOWN ACK and
set the T-bit in the Chunk Flags to indicate that no TCB was
found. Otherwise,
6) If the packet contains a SHUTDOWN COMPLETE chunk, the receiver
should silently discard the packet and take no further action.
Otherwise,
7) If the packet contains a "Stale cookie" ERROR or a COOKIE ACK the
SCTP Packet should be silently discarded. Otherwise,
8) The receiver should respond to the sender of the OOTB packet with
an ABORT. When sending the ABORT, the receiver of the OOTB packet
MUST fill in the Verification Tag field of the outbound packet
with the value found in the Verification Tag field of the OOTB
packet and set the T-bit in the Chunk Flags to indicate that no
TCB was found. After sending this ABORT, the receiver of the OOTB
packet shall discard the OOTB packet and take no further action.
8.5 Verification Tag
The Verification Tag rules defined in this section apply when sending
or receiving SCTP packets which do not contain an INIT, SHUTDOWN
COMPLETE, COOKIE ECHO (see Section 5.1), ABORT or SHUTDOWN ACK chunk.
The rules for sending and receiving SCTP packets containing one of
these chunk types are discussed separately in Section 8.5.1.
When sending an SCTP packet, the endpoint MUST fill in the
Verification Tag field of the outbound packet with the tag value in
the Initiate Tag parameter of the INIT or INIT ACK received from its
peer.
When receiving an SCTP packet, the endpoint MUST ensure that the
value in the Verification Tag field of the received SCTP packet
matches its own Tag. If the received Verification Tag value does not
match the receiver's own tag value, the receiver shall silently
discard the packet and shall not process it any further except for
those cases listed in Section 8.5.1 below.
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RFC 2960 Stream Control Transmission Protocol October 20008.5.1 Exceptions in Verification Tag Rules
A) Rules for packet carrying INIT:
- The sender MUST set the Verification Tag of the packet to 0.
- When an endpoint receives an SCTP packet with the Verification
Tag set to 0, it should verify that the packet contains only an
INIT chunk. Otherwise, the receiver MUST silently discard the
packet.
B) Rules for packet carrying ABORT:
- The endpoint shall always fill in the Verification Tag field of
the outbound packet with the destination endpoint's tag value
if it is known.
- If the ABORT is sent in response to an OOTB packet, the
endpoint MUST follow the procedure described in Section 8.4.
- The receiver MUST accept the packet if the Verification Tag
matches either its own tag, OR the tag of its peer. Otherwise,
the receiver MUST silently discard the packet and take no
further action.
C) Rules for packet carrying SHUTDOWN COMPLETE:
- When sending a SHUTDOWN COMPLETE, if the receiver of the
SHUTDOWN ACK has a TCB then the destination endpoint's tag MUST
be used. Only where no TCB exists should the sender use the
Verification Tag from the SHUTDOWN ACK.
- The receiver of a SHUTDOWN COMPLETE shall accept the packet if
the Verification Tag field of the packet matches its own tag OR
it is set to its peer's tag and the T bit is set in the Chunk
Flags. Otherwise, the receiver MUST silently discard the packet
and take no further action. An endpoint MUST ignore the
SHUTDOWN COMPLETE if it is not in the SHUTDOWN-ACK-SENT state.
D) Rules for packet carrying a COOKIE ECHO
- When sending a COOKIE ECHO, the endpoint MUST use the value of
the Initial Tag received in the INIT ACK.
- The receiver of a COOKIE ECHO follows the procedures in Section5.
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RFC 2960 Stream Control Transmission Protocol October 2000
E) Rules for packet carrying a SHUTDOWN ACK
- If the receiver is in COOKIE-ECHOED or COOKIE-WAIT state the
procedures in section 8.4 SHOULD be followed, in other words it
should be treated as an Out Of The Blue packet.
9. Termination of Association
An endpoint should terminate its association when it exits from
service. An association can be terminated by either abort or
shutdown. An abort of an association is abortive by definition in
that any data pending on either end of the association is discarded
and not delivered to the peer. A shutdown of an association is
considered a graceful close where all data in queue by either
endpoint is delivered to the respective peers. However, in the case
of a shutdown, SCTP does not support a half-open state (like TCP)
wherein one side may continue sending data while the other end is
closed. When either endpoint performs a shutdown, the association on
each peer will stop accepting new data from its user and only deliver
data in queue at the time of sending or receiving the SHUTDOWN chunk.
9.1 Abort of an Association
When an endpoint decides to abort an existing association, it shall
send an ABORT chunk to its peer endpoint. The sender MUST fill in
the peer's Verification Tag in the outbound packet and MUST NOT
bundle any DATA chunk with the ABORT.
An endpoint MUST NOT respond to any received packet that contains an
ABORT chunk (also see Section 8.4).
An endpoint receiving an ABORT shall apply the special Verification
Tag check rules described in Section 8.5.1.
After checking the Verification Tag, the receiving endpoint shall
remove the association from its record, and shall report the
termination to its upper layer.
9.2 Shutdown of an Association
Using the SHUTDOWN primitive (see Section 10.1), the upper layer of
an endpoint in an association can gracefully close the association.
This will allow all outstanding DATA chunks from the peer of the
shutdown initiator to be delivered before the association terminates.
Upon receipt of the SHUTDOWN primitive from its upper layer, the
endpoint enters SHUTDOWN-PENDING state and remains there until all
outstanding data has been acknowledged by its peer. The endpoint
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RFC 2960 Stream Control Transmission Protocol October 2000
accepts no new data from its upper layer, but retransmits data to the
far end if necessary to fill gaps.
Once all its outstanding data has been acknowledged, the endpoint
shall send a SHUTDOWN chunk to its peer including in the Cumulative
TSN Ack field the last sequential TSN it has received from the peer.
It shall then start the T2-shutdown timer and enter the SHUTDOWN-SENT
state. If the timer expires, the endpoint must re-send the SHUTDOWN
with the updated last sequential TSN received from its peer.
The rules in Section 6.3 MUST be followed to determine the proper
timer value for T2-shutdown. To indicate any gaps in TSN, the
endpoint may also bundle a SACK with the SHUTDOWN chunk in the same
SCTP packet.
An endpoint should limit the number of retransmissions of the
SHUTDOWN chunk to the protocol parameter 'Association.Max.Retrans'.
If this threshold is exceeded the endpoint should destroy the TCB and
MUST report the peer endpoint unreachable to the upper layer (and
thus the association enters the CLOSED state). The reception of any
packet from its peer (i.e. as the peer sends all of its queued DATA
chunks) should clear the endpoint's retransmission count and restart
the T2-Shutdown timer, giving its peer ample opportunity to transmit
all of its queued DATA chunks that have not yet been sent.
Upon the reception of the SHUTDOWN, the peer endpoint shall
- enter the SHUTDOWN-RECEIVED state,
- stop accepting new data from its SCTP user
- verify, by checking the Cumulative TSN Ack field of the chunk,
that all its outstanding DATA chunks have been received by the
SHUTDOWN sender.
Once an endpoint as reached the SHUTDOWN-RECEIVED state it MUST NOT
send a SHUTDOWN in response to a ULP request, and should discard
subsequent SHUTDOWN chunks.
If there are still outstanding DATA chunks left, the SHUTDOWN
receiver shall continue to follow normal data transmission procedures
defined in Section 6 until all outstanding DATA chunks are
acknowledged; however, the SHUTDOWN receiver MUST NOT accept new data
from its SCTP user.
While in SHUTDOWN-SENT state, the SHUTDOWN sender MUST immediately
respond to each received packet containing one or more DATA chunk(s)
with a SACK, a SHUTDOWN chunk, and restart the T2-shutdown timer. If
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it has no more outstanding DATA chunks, the SHUTDOWN receiver shall
send a SHUTDOWN ACK and start a T2-shutdown timer of its own,
entering the SHUTDOWN-ACK-SENT state. If the timer expires, the
endpoint must re-send the SHUTDOWN ACK.
The sender of the SHUTDOWN ACK should limit the number of
retransmissions of the SHUTDOWN ACK chunk to the protocol parameter '
Association.Max.Retrans'. If this threshold is exceeded the endpoint
should destroy the TCB and may report the peer endpoint unreachable
to the upper layer (and thus the association enters the CLOSED
state).
Upon the receipt of the SHUTDOWN ACK, the SHUTDOWN sender shall stop
the T2-shutdown timer, send a SHUTDOWN COMPLETE chunk to its peer,
and remove all record of the association.
Upon reception of the SHUTDOWN COMPLETE chunk the endpoint will
verify that it is in SHUTDOWN-ACK-SENT state, if it is not the chunk
should be discarded. If the endpoint is in the SHUTDOWN-ACK-SENT
state the endpoint should stop the T2-shutdown timer and remove all
knowledge of the association (and thus the association enters the
CLOSED state).
An endpoint SHOULD assure that all its outstanding DATA chunks have
been acknowledged before initiating the shutdown procedure.
An endpoint should reject any new data request from its upper layer
if it is in SHUTDOWN-PENDING, SHUTDOWN-SENT, SHUTDOWN-RECEIVED, or
SHUTDOWN-ACK-SENT state.
If an endpoint is in SHUTDOWN-ACK-SENT state and receives an INIT
chunk (e.g., if the SHUTDOWN COMPLETE was lost) with source and
destination transport addresses (either in the IP addresses or in the
INIT chunk) that belong to this association, it should discard the
INIT chunk and retransmit the SHUTDOWN ACK chunk.
Note: Receipt of an INIT with the same source and destination IP
addresses as used in transport addresses assigned to an endpoint but
with a different port number indicates the initialization of a
separate association.
The sender of the INIT or COOKIE ECHO should respond to the receipt
of a SHUTDOWN-ACK with a stand-alone SHUTDOWN COMPLETE in an SCTP
packet with the Verification Tag field of its common header set to
the same tag that was received in the SHUTDOWN ACK packet. This is
considered an Out of the Blue packet as defined in Section 8.4. The
sender of the INIT lets T1-init continue running and remains in the
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COOKIE-WAIT or COOKIE-ECHOED state. Normal T1-init timer expiration
will cause the INIT or COOKIE chunk to be retransmitted and thus
start a new association.
If a SHUTDOWN is received in COOKIE WAIT or COOKIE ECHOED states the
SHUTDOWN chunk SHOULD be silently discarded.
If an endpoint is in SHUTDOWN-SENT state and receives a SHUTDOWN
chunk from its peer, the endpoint shall respond immediately with a
SHUTDOWN ACK to its peer, and move into a SHUTDOWN-ACK-SENT state
restarting its T2-shutdown timer.
If an endpoint is in the SHUTDOWN-ACK-SENT state and receives a
SHUTDOWN ACK, it shall stop the T2-shutdown timer, send a SHUTDOWN
COMPLETE chunk to its peer, and remove all record of the association.
10. Interface with Upper Layer
The Upper Layer Protocols (ULP) shall request for services by passing
primitives to SCTP and shall receive notifications from SCTP for
various events.
The primitives and notifications described in this section should be
used as a guideline for implementing SCTP. The following functional
description of ULP interface primitives is shown for illustrative
purposes. Different SCTP implementations may have different ULP
interfaces. However, all SCTPs must provide a certain minimum set of
services to guarantee that all SCTP implementations can support the
same protocol hierarchy.
10.1 ULP-to-SCTP
The following sections functionally characterize a ULP/SCTP
interface. The notation used is similar to most procedure or
function calls in high level languages.
The ULP primitives described below specify the basic functions the
SCTP must perform to support inter-process communication. Individual
implementations must define their own exact format, and may provide
combinations or subsets of the basic functions in single calls.
A) Initialize
Format: INITIALIZE ([local port], [local eligible address list]) ->
local SCTP instance name
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This primitive allows SCTP to initialize its internal data structures
and allocate necessary resources for setting up its operation
environment. Once SCTP is initialized, ULP can communicate directly
with other endpoints without re-invoking this primitive.
SCTP will return a local SCTP instance name to the ULP.
Mandatory attributes:
None.
Optional attributes:
The following types of attributes may be passed along with the
primitive:
o local port - SCTP port number, if ULP wants it to be specified;
o local eligible address list - An address list that the local SCTP
endpoint should bind. By default, if an address list is not
included, all IP addresses assigned to the host should be used by
the local endpoint.
IMPLEMENTATION NOTE: If this optional attribute is supported by an
implementation, it will be the responsibility of the implementation
to enforce that the IP source address field of any SCTP packets sent
out by this endpoint contains one of the IP addresses indicated in
the local eligible address list.
B) Associate
Format: ASSOCIATE(local SCTP instance name, destination transport addr,
outbound stream count)
-> association id [,destination transport addr list] [,outbound stream
count]
This primitive allows the upper layer to initiate an association to a
specific peer endpoint.
The peer endpoint shall be specified by one of the transport
addresses which defines the endpoint (see Section 1.4). If the local
SCTP instance has not been initialized, the ASSOCIATE is considered
an error.
An association id, which is a local handle to the SCTP association,
will be returned on successful establishment of the association. If
SCTP is not able to open an SCTP association with the peer endpoint,
an error is returned.
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Other association parameters may be returned, including the complete
destination transport addresses of the peer as well as the outbound
stream count of the local endpoint. One of the transport address
from the returned destination addresses will be selected by the local
endpoint as default primary path for sending SCTP packets to this
peer. The returned "destination transport addr list" can be used by
the ULP to change the default primary path or to force sending a
packet to a specific transport address.
IMPLEMENTATION NOTE: If ASSOCIATE primitive is implemented as a
blocking function call, the ASSOCIATE primitive can return
association parameters in addition to the association id upon
successful establishment. If ASSOCIATE primitive is implemented as a
non-blocking call, only the association id shall be returned and
association parameters shall be passed using the COMMUNICATION UP
notification.
Mandatory attributes:
o local SCTP instance name - obtained from the INITIALIZE operation.
o destination transport addr - specified as one of the transport
addresses of the peer endpoint with which the association is to be
established.
o outbound stream count - the number of outbound streams the ULP
would like to open towards this peer endpoint.
Optional attributes:
None.
C) Shutdown
Format: SHUTDOWN(association id)
-> result
Gracefully closes an association. Any locally queued user data will
be delivered to the peer. The association will be terminated only
after the peer acknowledges all the SCTP packets sent. A success
code will be returned on successful termination of the association.
If attempting to terminate the association results in a failure, an
error code shall be returned.
Mandatory attributes:
o association id - local handle to the SCTP association
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Optional attributes:
None.
D) Abort
Format: ABORT(association id [, cause code])
-> result
Ungracefully closes an association. Any locally queued user data
will be discarded and an ABORT chunk is sent to the peer. A success
code will be returned on successful abortion of the association. If
attempting to abort the association results in a failure, an error
code shall be returned.
Mandatory attributes:
o association id - local handle to the SCTP association
Optional attributes:
o cause code - reason of the abort to be passed to the peer.
None.
E) Send
Format: SEND(association id, buffer address, byte count [,context]
[,stream id] [,life time] [,destination transport address]
[,unorder flag] [,no-bundle flag] [,payload protocol-id] )
-> result
This is the main method to send user data via SCTP.
Mandatory attributes:
o association id - local handle to the SCTP association
o buffer address - the location where the user message to be
transmitted is stored;
o byte count - The size of the user data in number of bytes;
Optional attributes:
o context - an optional 32 bit integer that will be carried in the
sending failure notification to the ULP if the transportation of
this User Message fails.
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o stream id - to indicate which stream to send the data on. If not
specified, stream 0 will be used.
o life time - specifies the life time of the user data. The user
data will not be sent by SCTP after the life time expires. This
parameter can be used to avoid efforts to transmit stale user
messages. SCTP notifies the ULP if the data cannot be initiated
to transport (i.e. sent to the destination via SCTP's send
primitive) within the life time variable. However, the user data
will be transmitted if SCTP has attempted to transmit a chunk
before the life time expired.
IMPLEMENTATION NOTE: In order to better support the data lifetime
option, the transmitter may hold back the assigning of the TSN number
to an outbound DATA chunk to the last moment. And, for
implementation simplicity, once a TSN number has been assigned the
sender should consider the send of this DATA chunk as committed,
overriding any lifetime option attached to the DATA chunk.
o destination transport address - specified as one of the
destination transport addresses of the peer endpoint to which this
packet should be sent. Whenever possible, SCTP should use this
destination transport address for sending the packets, instead of
the current primary path.
o unorder flag - this flag, if present, indicates that the user
would like the data delivered in an unordered fashion to the peer
(i.e., the U flag is set to 1 on all DATA chunks carrying this
message).
o no-bundle flag - instructs SCTP not to bundle this user data with
other outbound DATA chunks. SCTP MAY still bundle even when this
flag is present, when faced with network congestion.
o payload protocol-id - A 32 bit unsigned integer that is to be
passed to the peer indicating the type of payload protocol data
being transmitted. This value is passed as opaque data by SCTP.
F) Set Primary
Format: SETPRIMARY(association id, destination transport address,
[source transport address] )
-> result
Instructs the local SCTP to use the specified destination transport
address as primary path for sending packets.
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The result of attempting this operation shall be returned. If the
specified destination transport address is not present in the
"destination transport address list" returned earlier in an associate
command or communication up notification, an error shall be returned.
Mandatory attributes:
o association id - local handle to the SCTP association
o destination transport address - specified as one of the transport
addresses of the peer endpoint, which should be used as primary
address for sending packets. This overrides the current primary
address information maintained by the local SCTP endpoint.
Optional attributes:
o source transport address - optionally, some implementations may
allow you to set the default source address placed in all outgoing
IP datagrams.
G) Receive
Format: RECEIVE(association id, buffer address, buffer size
[,stream id])
-> byte count [,transport address] [,stream id] [,stream sequence
number] [,partial flag] [,delivery number] [,payload protocol-id]
This primitive shall read the first user message in the SCTP in-queue
into the buffer specified by ULP, if there is one available. The
size of the message read, in bytes, will be returned. It may,
depending on the specific implementation, also return other
information such as the sender's address, the stream id on which it
is received, whether there are more messages available for retrieval,
etc. For ordered messages, their stream sequence number may also be
returned.
Depending upon the implementation, if this primitive is invoked when
no message is available the implementation should return an
indication of this condition or should block the invoking process
until data does become available.
Mandatory attributes:
o association id - local handle to the SCTP association
o buffer address - the memory location indicated by the ULP to store
the received message.
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o buffer size - the maximum size of data to be received, in bytes.
Optional attributes:
o stream id - to indicate which stream to receive the data on.
o stream sequence number - the stream sequence number assigned by
the sending SCTP peer.
o partial flag - if this returned flag is set to 1, then this
Receive contains a partial delivery of the whole message. When
this flag is set, the stream id and stream sequence number MUST
accompany this receive. When this flag is set to 0, it indicates
that no more deliveries will be received for this stream sequence
number.
o payload protocol-id - A 32 bit unsigned integer that is received
from the peer indicating the type of payload protocol of the
received data. This value is passed as opaque data by SCTP.
H) Status
Format: STATUS(association id)
-> status data
This primitive should return a data block containing the following
information:
association connection state,
destination transport address list,
destination transport address reachability states,
current receiver window size,
current congestion window sizes,
number of unacknowledged DATA chunks,
number of DATA chunks pending receipt,
primary path,
most recent SRTT on primary path,
RTO on primary path,
SRTT and RTO on other destination addresses, etc.
Mandatory attributes:
o association id - local handle to the SCTP association
Optional attributes:
None.
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I) Change Heartbeat
Format: CHANGEHEARTBEAT(association id, destination transport address,
new state [,interval])
-> result
Instructs the local endpoint to enable or disable heartbeat on the
specified destination transport address.
The result of attempting this operation shall be returned.
Note: Even when enabled, heartbeat will not take place if the
destination transport address is not idle.
Mandatory attributes:
o association id - local handle to the SCTP association
o destination transport address - specified as one of the transport
addresses of the peer endpoint.
o new state - the new state of heartbeat for this destination
transport address (either enabled or disabled).
Optional attributes:
o interval - if present, indicates the frequency of the heartbeat if
this is to enable heartbeat on a destination transport address.
This value is added to the RTO of the destination transport
address. This value, if present, effects all destinations.
J) Request HeartBeat
Format: REQUESTHEARTBEAT(association id, destination transport
address)
-> result
Instructs the local endpoint to perform a HeartBeat on the specified
destination transport address of the given association. The returned
result should indicate whether the transmission of the HEARTBEAT
chunk to the destination address is successful.
Mandatory attributes:
o association id - local handle to the SCTP association
o destination transport address - the transport address of the
association on which a heartbeat should be issued.
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K) Get SRTT Report
Format: GETSRTTREPORT(association id, destination transport address)
-> srtt result
Instructs the local SCTP to report the current SRTT measurement on
the specified destination transport address of the given association.
The returned result can be an integer containing the most recent SRTT
in milliseconds.
Mandatory attributes:
o association id - local handle to the SCTP association
o destination transport address - the transport address of the
association on which the SRTT measurement is to be reported.
L) Set Failure Threshold
Format: SETFAILURETHRESHOLD(association id, destination transport
address, failure threshold)
-> result
This primitive allows the local SCTP to customize the reachability
failure detection threshold 'Path.Max.Retrans' for the specified
destination address.
Mandatory attributes:
o association id - local handle to the SCTP association
o destination transport address - the transport address of the
association on which the failure detection threshold is to be set.
o failure threshold - the new value of 'Path.Max.Retrans' for the
destination address.
M) Set Protocol Parameters
Format: SETPROTOCOLPARAMETERS(association id, [,destination transport
address,] protocol parameter list)
-> result
This primitive allows the local SCTP to customize the protocol
parameters.
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Mandatory attributes:
o association id - local handle to the SCTP association
o protocol parameter list - The specific names and values of the
protocol parameters (e.g., Association.Max.Retrans [see Section14]) that the SCTP user wishes to customize.
Optional attributes:
o destination transport address - some of the protocol parameters
may be set on a per destination transport address basis.
N) Receive unsent message
Format: RECEIVE_UNSENT(data retrieval id, buffer address, buffer size
[,stream id] [, stream sequence number] [,partial flag]
[,payload protocol-id])
o data retrieval id - The identification passed to the ULP in the
failure notification.
o buffer address - the memory location indicated by the ULP to store
the received message.
o buffer size - the maximum size of data to be received, in bytes.
Optional attributes:
o stream id - this is a return value that is set to indicate
which stream the data was sent to.
o stream sequence number - this value is returned indicating
the stream sequence number that was associated with the message.
o partial flag - if this returned flag is set to 1, then this
message is a partial delivery of the whole message. When
this flag is set, the stream id and stream sequence number MUST
accompany this receive. When this flag is set to 0, it indicates
that no more deliveries will be received for this stream sequence
number.
o payload protocol-id - The 32 bit unsigned integer that was sent to
be sent to the peer indicating the type of payload protocol of the
received data.
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O) Receive unacknowledged message
Format: RECEIVE_UNACKED(data retrieval id, buffer address, buffer size,
[,stream id] [, stream sequence number] [,partial flag]
[,payload protocol-id])
o data retrieval id - The identification passed to the ULP in the
failure notification.
o buffer address - the memory location indicated by the ULP to store
the received message.
o buffer size - the maximum size of data to be received, in bytes.
Optional attributes:
o stream id - this is a return value that is set to indicate which
stream the data was sent to.
o stream sequence number - this value is returned indicating the
stream sequence number that was associated with the message.
o partial flag - if this returned flag is set to 1, then this
message is a partial delivery of the whole message. When this
flag is set, the stream id and stream sequence number MUST
accompany this receive. When this flag is set to 0, it indicates
that no more deliveries will be received for this stream sequence
number.
o payload protocol-id - The 32 bit unsigned integer that was sent to
be sent to the peer indicating the type of payload protocol of the
received data.
P) Destroy SCTP instance
Format: DESTROY(local SCTP instance name)
o local SCTP instance name - this is the value that was passed to
the application in the initialize primitive and it indicates which
SCTP instance to be destroyed.
10.2 SCTP-to-ULP
It is assumed that the operating system or application environment
provides a means for the SCTP to asynchronously signal the ULP
process. When SCTP does signal an ULP process, certain information
is passed to the ULP.
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IMPLEMENTATION NOTE: In some cases this may be done through a
separate socket or error channel.
A) DATA ARRIVE notification
SCTP shall invoke this notification on the ULP when a user message is
successfully received and ready for retrieval.
The following may be optionally be passed with the notification:
o association id - local handle to the SCTP association
o stream id - to indicate which stream the data is received on.
B) SEND FAILURE notification
If a message can not be delivered SCTP shall invoke this notification
on the ULP.
The following may be optionally be passed with the notification:
o association id - local handle to the SCTP association
o data retrieval id - an identification used to retrieve unsent and
unacknowledged data.
o cause code - indicating the reason of the failure, e.g., size too
large, message life-time expiration, etc.
o context - optional information associated with this message (see D
in Section 10.1).
C) NETWORK STATUS CHANGE notification
When a destination transport address is marked inactive (e.g., when
SCTP detects a failure), or marked active (e.g., when SCTP detects a
recovery), SCTP shall invoke this notification on the ULP.
The following shall be passed with the notification:
o association id - local handle to the SCTP association
o destination transport address - This indicates the destination
transport address of the peer endpoint affected by the change;
o new-status - This indicates the new status.
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D) COMMUNICATION UP notification
This notification is used when SCTP becomes ready to send or receive
user messages, or when a lost communication to an endpoint is
restored.
IMPLEMENTATION NOTE: If ASSOCIATE primitive is implemented as a
blocking function call, the association parameters are returned as a
result of the ASSOCIATE primitive itself. In that case,
COMMUNICATION UP notification is optional at the association
initiator's side.
The following shall be passed with the notification:
o association id - local handle to the SCTP association
o status - This indicates what type of event has occurred
o destination transport address list - the complete set of transport
addresses of the peer
o outbound stream count - the maximum number of streams allowed to
be used in this association by the ULP
o inbound stream count - the number of streams the peer endpoint has
requested with this association (this may not be the same number
as 'outbound stream count').
E) COMMUNICATION LOST notification
When SCTP loses communication to an endpoint completely (e.g., via
Heartbeats) or detects that the endpoint has performed an abort
operation, it shall invoke this notification on the ULP.
The following shall be passed with the notification:
o association id - local handle to the SCTP association
o status - This indicates what type of event has occurred; The status
may indicate a failure OR a normal termination event
occurred in response to a shutdown or abort request.
The following may be passed with the notification:
o data retrieval id - an identification used to retrieve unsent and
unacknowledged data.
o last-acked - the TSN last acked by that peer endpoint;
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o last-sent - the TSN last sent to that peer endpoint;
F) COMMUNICATION ERROR notification
When SCTP receives an ERROR chunk from its peer and decides to notify
its ULP, it can invoke this notification on the ULP.
The following can be passed with the notification:
o association id - local handle to the SCTP association
o error info - this indicates the type of error and optionally some
additional information received through the ERROR chunk.
G) RESTART notification
When SCTP detects that the peer has restarted, it may send this
notification to its ULP.
The following can be passed with the notification:
o association id - local handle to the SCTP association
H) SHUTDOWN COMPLETE notification
When SCTP completes the shutdown procedures (section 9.2) this
notification is passed to the upper layer.
The following can be passed with the notification:
o association id - local handle to the SCTP association
11. Security Considerations11.1 Security Objectives
As a common transport protocol designed to reliably carry time-
sensitive user messages, such as billing or signaling messages for
telephony services, between two networked endpoints, SCTP has the
following security objectives.
- availability of reliable and timely data transport services
- integrity of the user-to-user information carried by SCTP
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SCTP may potentially be used in a wide variety of risk situations.
It is important for operator(s) of systems running SCTP to analyze
their particular situations and decide on the appropriate counter-
measures.
Operators of systems running SCTP should consult [RFC2196] for
guidance in securing their site.
11.2.1 Countering Insider Attacks
The principles of [RFC2196] should be applied to minimize the risk of
theft of information or sabotage by insiders. Such procedures
include publication of security policies, control of access at the
physical, software, and network levels, and separation of services.
11.2.2 Protecting against Data Corruption in the Network
Where the risk of undetected errors in datagrams delivered by the
lower layer transport services is considered to be too great,
additional integrity protection is required. If this additional
protection were provided in the application-layer, the SCTP header
would remain vulnerable to deliberate integrity attacks. While the
existing SCTP mechanisms for detection of packet replays are
considered sufficient for normal operation, stronger protections are
needed to protect SCTP when the operating environment contains
significant risk of deliberate attacks from a sophisticated
adversary.
In order to promote software code-reuse, to avoid re-inventing the
wheel, and to avoid gratuitous complexity to SCTP, the IP
Authentication Header [RFC2402] SHOULD be used when the threat
environment requires stronger integrity protections, but does not
require confidentiality.
A widely implemented BSD Sockets API extension exists for
applications to request IP security services, such as AH or ESP from
an operating system kernel. Applications can use such an API to
request AH whenever AH use is appropriate.
11.2.3 Protecting Confidentiality
In most cases, the risk of breach of confidentiality applies to the
signaling data payload, not to the SCTP or lower-layer protocol
overheads. If that is true, encryption of the SCTP user data only
might be considered. As with the supplementary checksum service,
user data encryption MAY be performed by the SCTP user application.
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Alternately, the user application MAY use an implementation-specific
API to request that the IP Encapsulating Security Payload (ESP)
[RFC2406] be used to provide confidentiality and integrity.
Particularly for mobile users, the requirement for confidentiality
might include the masking of IP addresses and ports. In this case
ESP SHOULD be used instead of application-level confidentiality. If
ESP is used to protect confidentiality of SCTP traffic, an ESP
cryptographic transform that includes cryptographic integrity
protection MUST be used, because if there is a confidentiality threat
there will also be a strong integrity threat.
Whenever ESP is in use, application-level encryption is not generally
required.
Regardless of where confidentiality is provided, the ISAKMP [RFC2408]
and the Internet Key Exchange (IKE) [RFC2409] SHOULD be used for key
management.
Operators should consult [RFC2401] for more information on the
security services available at and immediately above the Internet
Protocol layer.
11.2.4 Protecting against Blind Denial of Service Attacks
A blind attack is one where the attacker is unable to intercept or
otherwise see the content of data flows passing to and from the
target SCTP node. Blind denial of service attacks may take the form
of flooding, masquerade, or improper monopolization of services.
11.2.4.1 Flooding
The objective of flooding is to cause loss of service and incorrect
behavior at target systems through resource exhaustion, interference
with legitimate transactions, and exploitation of buffer-related
software bugs. Flooding may be directed either at the SCTP node or
at resources in the intervening IP Access Links or the Internet.
Where the latter entities are the target, flooding will manifest
itself as loss of network services, including potentially the breach
of any firewalls in place.
In general, protection against flooding begins at the equipment
design level, where it includes measures such as:
- avoiding commitment of limited resources before determining that
the request for service is legitimate
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- giving priority to completion of processing in progress over the
acceptance of new work
- identification and removal of duplicate or stale queued requests
for service.
- not responding to unexpected packets sent to non-unicast
addresses.
Network equipment should be capable of generating an alarm and log if
a suspicious increase in traffic occurs. The log should provide
information such as the identity of the incoming link and source
address(es) used which will help the network or SCTP system operator
to take protective measures. Procedures should be in place for the
operator to act on such alarms if a clear pattern of abuse emerges.
The design of SCTP is resistant to flooding attacks, particularly in
its use of a four-way start-up handshake, its use of a cookie to
defer commitment of resources at the responding SCTP node until the
handshake is completed, and its use of a Verification Tag to prevent
insertion of extraneous packets into the flow of an established
association.
The IP Authentication Header and Encapsulating Security Payload might
be useful in reducing the risk of certain kinds of denial of service
attacks."
The use of the Host Name feature in the INIT chunk could be used to
flood a target DNS server. A large backlog of DNS queries, resolving
the Host Name received in the INIT chunk to IP addresses, could be
accomplished by sending INIT's to multiple hosts in a given domain.
In addition, an attacker could use the Host Name feature in an
indirect attack on a third party by sending large numbers of INITs to
random hosts containing the host name of the target. In addition to
the strain on DNS resources, this could also result in large numbers
of INIT ACKs being sent to the target. One method to protect against
this type of attack is to verify that the IP addresses received from
DNS include the source IP address of the original INIT. If the list
of IP addresses received from DNS does not include the source IP
address of the INIT, the endpoint MAY silently discard the INIT.
This last option will not protect against the attack against the DNS.
Stewart, et al. Standards Track [Page 117]

RFC 2960 Stream Control Transmission Protocol October 200011.2.4.2 Blind Masquerade
Masquerade can be used to deny service in several ways:
- by tying up resources at the target SCTP node to which the
impersonated node has limited access. For example, the target
node may by policy permit a maximum of one SCTP association with
the impersonated SCTP node. The masquerading attacker may attempt
to establish an association purporting to come from the
impersonated node so that the latter cannot do so when it requires
it.
- by deliberately allowing the impersonation to be detected, thereby
provoking counter-measures which cause the impersonated node to be
locked out of the target SCTP node.
- by interfering with an established association by inserting
extraneous content such as a SHUTDOWN request.
SCTP reduces the risk of blind masquerade attacks through IP spoofing
by use of the four-way startup handshake. Man-in-the-middle
masquerade attacks are discussed in Section 11.3 below. Because the
initial exchange is memoryless, no lockout mechanism is triggered by
blind masquerade attacks. In addition, the INIT ACK containing the
State Cookie is transmitted back to the IP address from which it
received the INIT. Thus the attacker would not receive the INIT ACK
containing the State Cookie. SCTP protects against insertion of
extraneous packets into the flow of an established association by use
of the Verification Tag.
Logging of received INIT requests and abnormalities such as
unexpected INIT ACKs might be considered as a way to detect patterns
of hostile activity. However, the potential usefulness of such
logging must be weighed against the increased SCTP startup processing
it implies, rendering the SCTP node more vulnerable to flooding
attacks. Logging is pointless without the establishment of operating
procedures to review and analyze the logs on a routine basis.
11.2.4.3 Improper Monopolization of Services
Attacks under this heading are performed openly and legitimately by
the attacker. They are directed against fellow users of the target
SCTP node or of the shared resources between the attacker and the
target node. Possible attacks include the opening of a large number
of associations between the attacker's node and the target, or
transfer of large volumes of information within a legitimately-
established association.
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RFC 2960 Stream Control Transmission Protocol October 2000
Policy limits should be placed on the number of associations per
adjoining SCTP node. SCTP user applications should be capable of
detecting large volumes of illegitimate or "no-op" messages within a
given association and either logging or terminating the association
as a result, based on local policy.
11.3 Protection against Fraud and Repudiation
The objective of fraud is to obtain services without authorization
and specifically without paying for them. In order to achieve this
objective, the attacker must induce the SCTP user application at the
target SCTP node to provide the desired service while accepting
invalid billing data or failing to collect it. Repudiation is a
related problem, since it may occur as a deliberate act of fraud or
simply because the repudiating party kept inadequate records of
service received.
Potential fraudulent attacks include interception and misuse of
authorizing information such as credit card numbers, blind masquerade
and replay, and man-in-the middle attacks which modify the packets
passing through a target SCTP association in real time.
The interception attack is countered by the confidentiality measures
discussed in Section 11.2.3 above.
Section 11.2.4.2 describes how SCTP is resistant to blind masquerade
attacks, as a result of the four-way startup handshake and the
Verification Tag. The Verification Tag and TSN together are
protections against blind replay attacks, where the replay is into an
existing association.
However, SCTP does not protect against man-in-the-middle attacks
where the attacker is able to intercept and alter the packets sent
and received in an association. For example, the INIT ACK will have
sufficient information sent on the wire for an adversary in the
middle to hijack an existing SCTP association. Where a significant
possibility of such attacks is seen to exist, or where possible
repudiation is an issue, the use of the IPSEC AH service is
recommended to ensure both the integrity and the authenticity of the
SCTP packets passed.
SCTP also provides no protection against attacks originating at or
beyond the SCTP node and taking place within the context of an
existing association. Prevention of such attacks should be covered
by appropriate security policies at the host site, as discussed in
Section 11.2.1.
Stewart, et al. Standards Track [Page 119]

RFC 2960 Stream Control Transmission Protocol October 200012. Recommended Transmission Control Block (TCB) Parameters
This section details a recommended set of parameters that should be
contained within the TCB for an implementation. This section is for
illustrative purposes and should not be deemed as requirements on an
implementation or as an exhaustive list of all parameters inside an
SCTP TCB. Each implementation may need its own additional parameters
for optimization.
12.1 Parameters necessary for the SCTP instance
Associations: A list of current associations and mappings to the data
consumers for each association. This may be in the
form of a hash table or other implementation dependent
structure. The data consumers may be process
identification information such as file descriptors,
named pipe pointer, or table pointers dependent on how
SCTP is implemented.
Secret Key: A secret key used by this endpoint to compute the MAC.
This SHOULD be a cryptographic quality random number
with a sufficient length. Discussion in [RFC1750] can
be helpful in selection of the key.
Address List: The list of IP addresses that this instance has bound.
This information is passed to one's peer(s) in INIT and
INIT ACK chunks.
SCTP Port: The local SCTP port number the endpoint is bound to.
12.2 Parameters necessary per association (i.e. the TCB)
Peer : Tag value to be sent in every packet and is received
Verification: in the INIT or INIT ACK chunk.
Tag :
My : Tag expected in every inbound packet and sent in the
Verification: INIT or INIT ACK chunk.
Tag :
State : A state variable indicating what state the association
: is in, i.e. COOKIE-WAIT, COOKIE-ECHOED, ESTABLISHED,
: SHUTDOWN-PENDING, SHUTDOWN-SENT, SHUTDOWN-RECEIVED,
: SHUTDOWN-ACK-SENT.
Note: No "CLOSED" state is illustrated since if a
association is "CLOSED" its TCB SHOULD be removed.
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RFC 2960 Stream Control Transmission Protocol October 2000
Peer : A list of SCTP transport addresses that the peer is
Transport : bound to. This information is derived from the INIT or
Address : INIT ACK and is used to associate an inbound packet
List : with a given association. Normally this information is
: hashed or keyed for quick lookup and access of the TCB.
Primary : This is the current primary destination transport
Path : address of the peer endpoint. It may also specify a
: source transport address on this endpoint.
Overall : The overall association error count.
Error Count :
Overall : The threshold for this association that if the Overall
Error : Error Count reaches will cause this association to be
Threshold : torn down.
Peer Rwnd : Current calculated value of the peer's rwnd.
Next TSN : The next TSN number to be assigned to a new DATA chunk.
: This is sent in the INIT or INIT ACK chunk to the peer
: and incremented each time a DATA chunk is assigned a
: TSN (normally just prior to transmit or during
: fragmentation).
Last Rcvd : This is the last TSN received in sequence. This value
TSN : is set initially by taking the peer's Initial TSN,
: received in the INIT or INIT ACK chunk, and
: subtracting one from it.
Mapping : An array of bits or bytes indicating which out of
Array : order TSN's have been received (relative to the
: Last Rcvd TSN). If no gaps exist, i.e. no out of order
: packets have been received, this array will be set to
: all zero. This structure may be in the form of a
: circular buffer or bit array.
Ack State : This flag indicates if the next received packet
: is to be responded to with a SACK. This is initialized
: to 0. When a packet is received it is incremented.
: If this value reaches 2 or more, a SACK is sent and the
: value is reset to 0. Note: This is used only when no
: DATA chunks are received out of order. When DATA chunks
: are out of order, SACK's are not delayed (see Section
: 6).
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RFC 2960 Stream Control Transmission Protocol October 2000
Inbound : An array of structures to track the inbound streams.
Streams : Normally including the next sequence number expected
: and possibly the stream number.
Outbound : An array of structures to track the outbound streams.
Streams : Normally including the next sequence number to
: be sent on the stream.
Reasm Queue : A re-assembly queue.
Local : The list of local IP addresses bound in to this
Transport : association.
Address :
List :
Association : The smallest PMTU discovered for all of the
PMTU : peer's transport addresses.
12.3 Per Transport Address Data
For each destination transport address in the peer's address list
derived from the INIT or INIT ACK chunk, a number of data elements
needs to be maintained including:
Error count : The current error count for this destination.
Error : Current error threshold for this destination i.e.
Threshold : what value marks the destination down if Error count
: reaches this value.
cwnd : The current congestion window.
ssthresh : The current ssthresh value.
RTO : The current retransmission timeout value.
SRTT : The current smoothed round trip time.
RTTVAR : The current RTT variation.
partial : The tracking method for increase of cwnd when in
bytes acked : congestion avoidance mode (see Section 6.2.2)
state : The current state of this destination, i.e. DOWN, UP,
: ALLOW-HB, NO-HEARTBEAT, etc.
PMTU : The current known path MTU.
Stewart, et al. Standards Track [Page 122]

RFC 2960 Stream Control Transmission Protocol October 2000
Per : A timer used by each destination.
Destination :
Timer :
RTO-Pending : A flag used to track if one of the DATA chunks sent to
this address is currently being used to compute a
RTT. If this flag is 0, the next DATA chunk sent to this
destination should be used to compute a RTT and this
flag should be set. Every time the RTT calculation
completes (i.e. the DATA chunk is SACK'd) clear this
flag.
last-time : The time this destination was last sent to. This can be
used : used to determine if a HEARTBEAT is needed.
12.4 General Parameters Needed
Out Queue : A queue of outbound DATA chunks.
In Queue : A queue of inbound DATA chunks.
13. IANA Considerations
This protocol will require port reservation like TCP for the use of
"well known" servers within the Internet. All current TCP ports
shall be automatically reserved in the SCTP port address space. New
requests should follow IANA's current mechanisms for TCP.
This protocol may also be extended through IANA in three ways:
-- through definition of additional chunk types,
-- through definition of additional parameter types, or
-- through definition of additional cause codes within
ERROR chunks
In the case where a particular ULP using SCTP desires to have its own
ports, the ULP should be responsible for registering with IANA for
getting its ports assigned.
13.1 IETF-defined Chunk Extension
The definition and use of new chunk types is an integral part of
SCTP. Thus, new chunk types are assigned by IANA through an IETF
Consensus action as defined in [RFC2434].
The documentation for a new chunk code type must include the
following information:
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RFC 2960 Stream Control Transmission Protocol October 2000
a) A long and short name for the new chunk type;
b) A detailed description of the structure of the chunk, which MUST
conform to the basic structure defined in Section 3.2;
c) A detailed definition and description of intended use of each
field within the chunk, including the chunk flags if any;
d) A detailed procedural description of the use of the new chunk type
within the operation of the protocol.
The last chunk type (255) is reserved for future extension if
necessary.
13.2 IETF-defined Chunk Parameter Extension
The assignment of new chunk parameter type codes is done through an
IETF Consensus action as defined in [RFC2434]. Documentation of the
chunk parameter MUST contain the following information:
a) Name of the parameter type.
b) Detailed description of the structure of the parameter field.
This structure MUST conform to the general type-length-value
format described in Section 3.2.1.
c) Detailed definition of each component of the parameter value.
d) Detailed description of the intended use of this parameter type,
and an indication of whether and under what circumstances multiple
instances of this parameter type may be found within the same
chunk.
13.3 IETF-defined Additional Error Causes
Additional cause codes may be allocated in the range 11 to 65535
through a Specification Required action as defined in [RFC2434].
Provided documentation must include the following information:
a) Name of the error condition.
b) Detailed description of the conditions under which an SCTP
endpoint should issue an ERROR (or ABORT) with this cause code.
c) Expected action by the SCTP endpoint which receives an ERROR (or
ABORT) chunk containing this cause code.
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RFC 2960 Stream Control Transmission Protocol October 2000
d) Detailed description of the structure and content of data fields
which accompany this cause code.
The initial word (32 bits) of a cause code parameter MUST conform to
the format shown in Section 3.3.10, i.e.:
-- first two bytes contain the cause code value
-- last two bytes contain length of the Cause Parameter.
13.4 Payload Protocol Identifiers
Except for value 0 which is reserved by SCTP to indicate an
unspecified payload protocol identifier in a DATA chunk, SCTP will
not be responsible for standardizing or verifying any payload
protocol identifiers; SCTP simply receives the identifier from the
upper layer and carries it with the corresponding payload data.
The upper layer, i.e., the SCTP user, SHOULD standardize any specific
protocol identifier with IANA if it is so desired. The use of any
specific payload protocol identifier is out of the scope of SCTP.
14. Suggested SCTP Protocol Parameter Values
The following protocol parameters are RECOMMENDED:
RTO.Initial - 3 seconds
RTO.Min - 1 second
RTO.Max - 60 seconds
RTO.Alpha - 1/8
RTO.Beta - 1/4
Valid.Cookie.Life - 60 seconds
Association.Max.Retrans - 10 attempts
Path.Max.Retrans - 5 attempts (per destination address)
Max.Init.Retransmits - 8 attempts
HB.interval - 30 seconds
IMPLEMENTATION NOTE: The SCTP implementation may allow ULP to
customize some of these protocol parameters (see Section 10).
Note: RTO.Min SHOULD be set as recommended above.
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RFC 2960 Stream Control Transmission Protocol October 2000
Appendix A: Explicit Congestion Notification
ECN (Ramakrishnan, K., Floyd, S., "Explicit Congestion Notification",
RFC 2481, January 1999) describes a proposed extension to IP that
details a method to become aware of congestion outside of datagram
loss. This is an optional feature that an implementation MAY choose
to add to SCTP. This appendix details the minor differences
implementers will need to be aware of if they choose to implement
this feature. In general RFC 2481 should be followed with the
following exceptions.
Negotiation:
RFC2481 details negotiation of ECN during the SYN and SYN-ACK stages
of a TCP connection. The sender of the SYN sets two bits in the TCP
flags, and the sender of the SYN-ACK sets only 1 bit. The reasoning
behind this is to assure both sides are truly ECN capable. For SCTP
this is not necessary. To indicate that an endpoint is ECN capable
an endpoint SHOULD add to the INIT and or INIT ACK chunk the TLV
reserved for ECN. This TLV contains no parameters, and thus has the
following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter Type = 32768 | Parameter Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ECN-Echo:
RFC 2481 details a specific bit for a receiver to send back in its
TCP acknowledgements to notify the sender of the Congestion
Experienced (CE) bit having arrived from the network. For SCTP this
same indication is made by including the ECNE chunk. This chunk
contains one data element, i.e. the lowest TSN associated with the IP
datagram marked with the CE bit, and looks as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Chunk Type=12 | Flags=00000000| Chunk Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lowest TSN Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Note: The ECNE is considered a Control chunk.
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RFC 2960 Stream Control Transmission Protocol October 2000
CWR:
RFC 2481 details a specific bit for a sender to send in the header of
its next outbound TCP segment to indicate to its peer that it has
reduced its congestion window. This is termed the CWR bit. For
SCTP the same indication is made by including the CWR chunk.
This chunk contains one data element, i.e. the TSN number that
was sent in the ECNE chunk. This element represents the lowest
TSN number in the datagram that was originally marked with the
CE bit.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Chunk Type=13 | Flags=00000000| Chunk Length = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lowest TSN Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Note: The CWR is considered a Control chunk.
Appendix B Alder 32 bit checksum calculation
The Adler-32 checksum calculation given in this appendix is copied from
[RFC1950].
Adler-32 is composed of two sums accumulated per byte: s1 is the sum
of all bytes, s2 is the sum of all s1 values. Both sums are done
modulo 65521. s1 is initialized to 1, s2 to zero. The Adler-32
checksum is stored as s2*65536 + s1 in network byte order.
The following C code computes the Adler-32 checksum of a data buffer.
It is written for clarity, not for speed. The sample code is in the
ANSI C programming language. Non C users may find it easier to read
with these hints:
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RFC 2960 Stream Control Transmission Protocol October 2000
Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Stewart, et al. Standards Track [Page 134]